"We here meet with a man altogether beyond the common standard, one in whom natural endowment and sedulous cultivation rivalled each other in the production of a true philosopher; nor do we hesitate to state our belief that, since Newton, Thomas Young stands unrivalled in the annals of British science." Such was the verdict of Principal Forbes on one who may not only be regarded as one of the founders of the undulatory theory of light, but who was among the first to apply the theory of elasticity to the strength of structures, while it is to him that we are indebted in the first instance for all we know of Egyptian hieroglyphics, and for the vast field of antiquarian research which the interpretation of these symbols has opened up.

Thomas Young was the son of Thomas and Sarah Young, and the eldest of ten children. His mother was a niece of the well-known physician, Dr. Richard Brocklesby, and both his father and mother were members of the Society of Friends, in whose principles all their children were very carefully trained. It was to the independence of character thus developed that Dr. Young attributed very much of the success which he afterwards attained. He was born at Milverton, in Somersetshire, on June 13, 1773. For the greater part of the first seven years of his life he lived with his maternal grandfather, Mr. Robert Davis, at Minehead, in Somersetshire. According to his own account, he could read with considerable fluency at the age of two, and, under the instructions of his aunt and a village schoolmistress, he had "read the Bible twice through, and also Watts's Hymns," before he attained the age of four. It may with reason be thought that both the schoolmistress and the aunt should have been severely reprimanded, and it is certain that their example is not to be commended; but Young's infantile constitution seems to have been proof against over-pressure, and before he was five years old he could recite the whole of Goldsmith's "Deserted Village," with scarcely a mistake. He commenced learning Latin before he was six, under the guidance of a Nonconformist minister, who also taught him to write. When not quite seven years of age he went to boarding-school, where he remained a year and a half; but he appears to have learned more by independent effort than under the guidance of his master, for privately he "had mastered the last rules of Walkinghame's 'Tutor's Assistant'" before reaching the middle of the book under the master's inspection. After leaving this school, he lived at home for six months, but frequently visited a neighbour who was a land surveyor, and at whose house he amused himself with philosophical instruments and scientific books, especially a "Dictionary of Arts and Sciences." When nearly nine he went to the school of Mr. Thompson, at Compton, in Dorsetshire, where he remained nearly four years, and read several Greek and Latin authors, as well as the elements of natural philosophy—the latter in books lent him by Mr. Jeffrey, the assistant-master. This Mr. Jeffrey appears to have been something of a mechanical genius, and he gave Young lessons in turning, drawing, bookbinding, and the grinding and preparation of colours. Before leaving this school, at the age of thirteen, Young had read six chapters of the Hebrew Bible.

During the school holidays the construction of a microscope occupied considerable time, and the reading of "Priestley on Air" turned Young's attention to the subject of chemistry. Having learned a little French, he succeeded, with the help of a schoolfellow, in gaining an elementary knowledge of Italian. After leaving school, he lived at home for some time, and devoted his energies mainly to Hebrew and to turning and telescope-making; but Eastern languages received a share of attention, and by the time he was fourteen he had read most of Sir William Jones's "Persian Grammar." He then went to Youngsbury, in Hertfordshire, and resided at the house of Mr. David Barclay, partly as companion and partly as classical tutor to Mr. Barclay's grandson, Hudson Gurney. This was the beginning of a friendship which lasted for life. Gurney was about a year and a half junior to Young, and for five years the boys studied together, reading the classical works which Young had previously studied at school. Before the end of these five years Young had gained more or less acquaintance with fourteen languages; but his studies were for a time delayed through a serious illness when he was little more than sixteen. To this illness his uncle, Dr. Brocklesby, referred in a letter, of which the following extract is interesting for several reasons:—

Recollect that the least slip (as who can be secure against error?) would in you, who seem in all things to set yourself above ordinary humanity, seem more monstrous or reprehensible than it might be in the generality of mankind. Your prudery about abstaining from the use of sugar on account of the negro trade, in any one else would be altogether ridiculous, but as long as the whole of your mind keeps free from spiritual pride or too much presumption in your facility of acquiring language, which is no more than the dross of knowledge, you may be indulged in such whims, till your mind becomes enlightened with more reason. My late excellent friend, Mr. Day, the author of 'Sandford and Merton,' abhorred the base traffic in negroes' lives as much as you can do, and even Mr. Granville Sharp, one of the earliest writers on the subject, has not done half as much service in the business as Mr. Day in the above work. And yet Mr. Day devoured daily as much sugar as I do; for he reasonably concluded that so great a system as the sugar-culture in the West Indies, where sixty millions of British property are employed, could never be affected either way by one or one hundred in the nation debarring themselves the reasonable use of it. Reformation must take its rise elsewhere, if ever there is a general mass of public virtue sufficient to resist such private interests. Read Locke with care, for he opens the avenues of knowledge, though he gives too little himself.

With respect to the sugar, no doubt very much may be said on Young's side of the question. It appears, however, that in his early manhood there was a good deal in his conduct which to-day would be regarded as priggish, though it was somewhat more in harmony with the spirit of his time.

He left Youngsbury at the age of nineteen, having read, besides his classical authors, the whole of Newton's "Principia" and "Opticks," and the systems of chemistry by Lavoisier and Nicholson, besides works on botany, medicine, mineralogy, and other scientific subjects. One of Young's peculiarities was the extraordinary neatness of his handwriting, and a translation in Greek iambics of Wolsey's farewell to Cromwell, which he sent, written very neatly on vellum, to his uncle, Dr. Brocklesby, attracted the attention of Mr. Burke, Dr. Charles Burney, and other classical scholars, so that when, a few months later, Young went to stay with his uncle in London, and was thrown into contact with some of the chief literary men of the day, he found that his fame as a scholar had preceded him. This neatness of his handwriting and his power of drawing were of great use in his researches on the Egyptian hieroglyphics. He had little faith in natural genius, but believed that anything could be accomplished by persevering application.

In the autumn of 1792 Young went to London for the purpose of studying medicine. He lived in lodgings in Westminster, and attended the Hunterian School of Anatomy. A year afterwards he entered St. Bartholomew's Hospital as a medical student. The notes which he took of the lectures were written sometimes in Latin, interspersed with Greek quotations, and not unfrequently with mathematical calculations, which may be assumed to have been made before the lecture commenced. During his school days he had paid some attention to geometrical optics, and had constructed a microscope and telescope. Now his attention was attracted to a far more delicate instrument—the eye itself. Young had learned how a telescope can be "focussed" so as to give clear images of objects more or less distant. Some such power of adjustment must be possessed by the eye, or it could never form distinct images of objects, whether at a distance of a foot or a mile. The apparently fibrous structure of the crystalline lens of the eye had been noticed and described by Leuwenhoeck; and Pemberton, a century before Young took up the subject, had suggested that the fibres were muscles, by the action of which the eye was "accommodated" for near or distant vision. In dissecting the eye of an ox Young thought he had discovered evidence confirmatory of this view, and the paper which he wrote on the subject was not only published in the "Philosophical Transactions," but secured his election as a Fellow of the Royal Society in June, 1794. This paper was important, not simply because it led to Young's election to the Royal Society, but mainly because it was his first published paper on optical subjects. Later on he showed incontestably, by exact measurements, that it is the crystalline lens which changes its form during adjustment; but he was wrong in supposing the fibres of the lens to be muscular. By carefully measuring the distance between the images of two candles formed by reflection from the cornea, he showed that the cornea experienced no change of form. His eyes were very prominent; and turning them so as to look very obliquely, he measured the length of the eye from back to front with a pair of compasses whose points were protected, pressing one point against the cornea, and the other between the back of the eye and the orbit, and showed that, when the eye was focussed for different distances, there was no change in the length of the axis. The crystalline lens was the only resource left whereby the accommodation could be effected. The accommodation is, in fact, brought about by the action of the ciliary muscle. The natural form of the lens is more convex than is consistent with distinct vision, except for very near objects. The tension of the suspensory ligament, which is attached to the front of the lens all round its edge, renders the anterior surface of the lens much less curved than it would naturally be. The ciliary muscle is a ring of muscular fibre attached to the ciliary process close to the circumference of the suspensory ligament. By its contraction it forms a smaller ring, and, diminishing the external diameter, it releases the tension of the suspensory ligament, thus allowing the crystalline lens to bulge out and adapt itself for the diverging rays coming from near objects. It is the exertion of contracting the ciliary muscle that constitutes the effort of which we are conscious when looking at very near objects. It was not, however, till long after the time of Dr. Young that this complicated action was fully made out, though the change of form of the anterior surface of the crystalline lens was discovered by the change in the image of a bright object formed by reflection.

In the spring of 1794 Young took a holiday tour in Cornwall, with Hudson Gurney, visiting on his way the Duke of Richmond, who was drinking the waters at Bath, under the advice of Dr. Brocklesby. In Cornwall, the mining machinery attracted his attention very much more than the natural beauties of the country. Towards the end of the summer he visited the Duke of Richmond at Goodwood, when the duke offered him the appointment of private secretary. He resolved, however, to continue his medical course, one of the reasons which he alleged being his regard for the Society of Friends, whose principles he considered inconsistent with the appointment of Private Secretary to the Master-General of the Ordnance.

The following winter he spent as a medical student at Edinburgh. Here he gave up the costume of the Society of Friends, and in many ways departed from their rules of conduct. He mingled freely with the university, attended the theatre, took lessons in dancing and playing the flute, and generally cultivated the habits of what is technically known as "society." Throughout this change in his life he retained his high moral principles as a guide of conduct, and appears to have acted from a firm conviction of what was right. At the same time, it must be admitted that the breaking down of barriers, however conventional they may be, is an operation attended in most cases by not a little danger. With Young, the progress of his scientific education may have been delayed on account of the new demands on his time; but besides the study of German, Spanish, and Italian, he appears to have read a considerable amount of general literature during his winter session in Edinburgh. The following summer he took a tour on horseback through the Highlands, taking with him his flute, drawing materials, spirits for preserving insects, boards for drying plants, paper and twine for packing up minerals, and a thermometer; but the geological hammer does not then appear to have been regarded as an essential to the equipment of a philosopher. At Aberdeen he stayed for three days, and reported thus on the university:—

Some of the professors are capable of raising a university to celebrity, especially Copeland and Ogilvie; but the division and proximity of the two universities (King's College and Marischal College) is not favourable to the advancement of learning; besides, the lectures are all, or mostly, given at the same hour, and the same professor continues to instruct a class for four years in the different branches. Were the colleges united, and the internal regulations of the system new modelled, the cheapness of the place, the number of small bursaries for poor or distinguished students, and the merit of the instructors, might make this university a very respectable seminary in some branches of science. The fee to a professor for a five-months' session is only a guinea and a half. I was delighted with the inspection of the rich store of mathematical and philosophical apparatus belonging to Professor Copeland of Marischal College, made in his own house, and partly with his own hands, finished with no less care than elegance; and tending to illustrate every branch of physics in the course of his lectures, which must be equally entertaining and instructive.

Before leaving the Highlands, Young visited Gordon Castle, where he stayed two days; and appears to have distinguished himself by the powers of endurance he exhibited in dancing reels. On leaving he writes: "I could almost have wished to break or dislocate a limb by chance, that I might be detained against my will; I do not recollect that I have ever passed my time more agreeably, or with a party that I thought more congenial to my own dispositions: and what would hardly be credited by many grave reasoners on life and manners, that a person who had spent the whole of his earlier years a recluse from the gay world, and a total stranger to all that was passing in the higher ranks of society, should feel himself more at home and more at ease in the most magnificent palace in the country than in the humblest dwelling with those whose birth was most similar to his own. Without enlarging on the duke's good sense and sincerity, the duchess's spirit and powers of conversation, Lady Madeline's liveliness and affability, Louisa's beauty and sweetness, Georgiana's naÏvetÉ and quickness of parts, young Sandy's good nature, I may say that I was truly sorry to part with every one of them."

Young seems not to have known at this time that it is an essential feature of true gentlefolk to dissipate all sense of constraint or uneasiness from those with whom they are brought into contact and that in this they can be readily distinguished from those who have wealth without breeding. The Duchess of Gordon gave Young an introduction to the Duke of Argyll, so, while travelling through the Western Highlands, he paid a visit to Inverary Castle, and "galloped over" the country with the duke's daughters. Speaking of these ladies, he says, "Lady Charlotte ... is to Lady Augusta what Venus is to Minerva; I suppose she wishes for no more. Both are goddesses."

On his return to the West of England, he visited the Coalbrook Dale Iron Works, when Mr. Reynolds told him "that before the war he had agreed with a man to make a flute a hundred and fifty feet long, and two and a half in diameter, to be blown by a steam-engine and played on by barrels."

On the 7th of the following October Young left London, and after spending six days on the voyage from Yarmouth to Hamburg, he reached GÖttingen on the 27th of the same month; two days afterwards he matriculated, and on November 3 he commenced his studies as a member of the university. He continued to take lessons in drawing, dancing, riding, and music, and commenced learning the clavichord. The English students at GÖttingen, in order to advance their German conversation, arranged to pay a fine whenever they spoke in English in one another's company. On Sundays it was usual for the professors to give entertainments to the students, though they seldom invited them to dinner or supper. "Indeed, they could not well afford, out of a fee of a louis or two, to give large entertainments; but the absence of the hospitality which prevails rather more in Britain, is compensated by the light in which the students are regarded; they are not the less, but perhaps the more, respected for being students, and indeed, they behave in general like gentlemen, much more so than in some other German universities."

At GÖttingen Young attended, in addition to his medical lectures, Spithler's lectures on the History and Constitution of the European States, Heyne on the History of the Ancient Arts, and Lichtenberg's course on Physics. Speaking of Blumenbach's lectures on Natural History, Young says, "He showed us yesterday a laborious treatise, with elegant plates, published in the beginning of this century at Wurzburg, which is a most singular specimen of credulity in affairs of natural history. Dr. Behringen used to torment the young men of a large school by obliging them to go out with him collecting petrifactions; and the young rogues, in revenge, spent a whole winter in counterfeiting specimens, which they buried in a hill which the good man meant to explore, and imposed them upon him for most wonderful lusus naturÆ. It is interesting in a metaphysical point of view to observe how the mind attempts to accommodate itself; in one case, where the boys had made the figure of a plant thick and clumsy, the doctor remarks the difference, and says that Nature seems to have restored to the plant in thickness that which she had taken away from its other dimensions."

On April 30, 1796, Young passed the examination for his medical degree at GÖttingen. The examination appears to have been entirely oral. It lasted between four and five hours. There were four examiners seated round a table provided "with cakes, sweetmeats, and wine, which helped to pass the time agreeably." They "were not very severe in exacting accurate answers." The subject he selected for his public discussion was the human voice, and he constructed a universal alphabet consisting of forty-seven letters, of which, however, very little is known. This study of sound laid the foundation, according to his own account, of his subsequent researches in the undulatory theory of light.

The autumn of 1796 Young spent in travelling in Germany; in the following February he returned to England, and was admitted a fellow-commoner of Emmanuel College, Cambridge. It is said that the Master, in introducing Young to the Tutors and other Fellows, said, "I have brought you a pupil qualified to read lectures to his tutors." Young's opinion of Cambridge, as compared with German universities, was favourable to the former; but as he had complained of the want of hospitality at GÖttingen, so in Cambridge he complained of the want of social intercourse between the senior members of the university and persons in statu pupillari. At that time there was no system of medical education in the university, and the statutes required that six years should elapse between the admission of a medical student and his taking the degree of M.B. Young appears to have attracted comparatively little attention as an undergraduate in college. He did not care to associate with other undergraduates, and had little opportunity of intercourse with the senior members of the university. He was still keeping terms at Cambridge when his uncle, Dr. Brocklesby, died. To Young he left the house in Norfolk Street, Park Lane, with the furniture, books, pictures, and prints, and about £10,000. In the summer of 1798 a slight accident at Cambridge compelled Young to keep to his rooms, and being thus forcibly deprived of his usual round of social intercourse, he returned to his favourite studies in physics. The most important result of this study was the establishment of the principle of interference in sound, which afforded the explanation of the phenomenon of "beats" in music, and which afterwards led up to the discovery of the interference of light—a discovery which Sir John Herschel characterized as "the key to all the more abstruse and puzzling properties of light, and which would alone have sufficed to place its author in the highest rank of scientific immortality, even were his other almost innumerable claims to such a distinction disregarded."

The principle of interference is briefly this: When two waves meet each other, it may happen that their crests coincide; in this case a wave will be formed equal in height (amplitude) to the sum of the heights of the two. At another point the crest of one wave may coincide with the hollow of another, and, as the waves pass, the height of the wave at this point will be the difference of the two heights, and if the waves are equal the point will remain stationary. If a rope be hung from the ceiling of a lofty room, and the lower end receive a jerk from the hand, a wave will travel up the rope, be reflected and reversed at the ceiling, and then descend. If another wave be then sent up, the two will meet, and their passing can be observed. It will then be seen that, if the waves are exactly equal, the point at which they meet will remain at rest during the whole time of transit. If a number of waves in succession be sent up the string, the motions of the hand being properly timed, the string will appear to be divided into a number of vibrating segments separated by stationary points, or nodes. These nodes are simply the points which remain at rest on account of the upward series of waves crossing the series which have been reflected at the top and are travelling downwards. The division of a vibrating string into nodes thus affords a simple example of the principle of interference. When a tuning-fork is vibrating there are certain hyperbolic lines along which the disturbance caused by one prong is exactly neutralized by that due to the other prong. If a large tuning-fork be struck and then held near the ear and slowly turned round, the positions of comparative silence will be readily perceived. If two notes are being sounded side by side, one consisting of two hundred vibrations per second and the other of two hundred and two, then, at any distant point, it is clear that the two sets of waves will arrive in the same condition, or "phase," twice in each second, and twice they will be in opposite conditions, and, if of the same intensity, will exactly destroy one another's effects, thus producing silence. Hence twice in the second there will be silence and twice there will be sound, the waves of which have double the amplitude due to either source, and hence the sound will have four times the intensity of either note by itself. Thus there will be two "beats" per second due to interference. Later on this principle was applied by Young to very many optical phenomena of which it afforded a complete explanation.

Young completed his last term of residence at Cambridge in December, 1799, and in the early part of 1800 he commenced practice as a physician at 48, Welbeck Street. In the following year he accepted the chair of Natural Philosophy in the Royal Institution, which had shortly before been founded, and soon afterwards, in conjunction with Davy, the Professor of Chemistry, he undertook the editing of the journals of the institution. This circumstance has already been alluded to in connection with Count Rumford, the founder of the institution. He lectured at the Royal Institution for two years only, when he resigned the chair in deference to the popular belief that a physician should give his attention wholly to his professional practice, whether he has any or not. This fear lest a scientific reputation should interfere with his success as a physician haunted him for many years, and sometimes prevented his undertaking scientific work, while at other times it led him to publish anonymously the results he obtained. This anonymous publication of scientific papers caused him great trouble afterwards in order to establish his claim to his own discoveries. Many of the articles which he contributed to the supplement to the fourth, fifth, and sixth editions of the "EncyclopÆdia Britannica" were anonymous, although the honorarium he received for this work was increased by 25 per cent. when he would allow his name to appear. The practical withdrawal of Young from the scientific world during sixteen years was a great loss to the progress of natural philosophy, while the absence of that suavity of manner when dealing with patients which is so essential to the success of a physician, prevented him from acquiring a valuable private practice. In fact, Young was too much of a philosopher in his behaviour to succeed as a physician; he thought too deeply before giving his opinion on a diagnosis, instead of appearing to know all about the subject before he commenced his examination, and this habit, which is essential to the philosopher, does not inspire confidence in the practitioner. His fondness for society rendered him unwilling to live within the means which his uncle had left him, supplemented by what his scientific work might bring, and it was not until his income had been considerably increased by an appointment under the Admiralty that he was willing to forego the possible increase of practice which might accrue by appearing to devote his whole attention to the subject of medicine. It was this fear of public opinion which caused him, in 1812, to decline the offer of the appointment of Secretary to the Royal Society, of which, in 1802, he accepted the office of Foreign Secretary.

Young's resignation of the chair of Natural Philosophy was, however, not a great loss to the Royal Institution; for the lecture audience there was essentially of a popular character, and Young cannot be considered to have been successful as a popular lecturer. His own early education had been too much derived from private reading for him to have become acquainted with the difficulties experienced by beginners of only average ability, and his lectures, while most valuable to those who already possessed a fair knowledge of the subjects, were ill adapted to the requirements of an unscientific audience. A syllabus of his course of lectures was published by Young in 1802, but it was not till 1807 that the complete course of sixty lectures was published in two quarto volumes. They were republished in 1845 in octavo, with references and notes by Professor Kelland. Among the subjects treated in these lectures are mechanics, including strength of materials, architecture and carpentry, clocks, drawing and modelling; hydrostatics and hydraulics; sound and musical instruments; optics, including vision and the physical nature of light; astronomy; geography; the essential properties of matter; heat; electricity and magnetism; climate, winds, and meteorology generally; vegetation and animal life, and the history of the preceding sciences. The lectures were followed by a most complete bibliography of the whole subject, including works in English, French, German, Italian, and Latin. The following is the syllabus of one lecture, and illustrates the diversity of the subjects dealt with:—

"On Drawing, Writing, and Measuring.

"Subjects preliminary to the study of practical mechanics; instrumental geometry; statics; passive strength; friction; drawing; outline; pen; pencil; chalks; crayons; Indian ink; water-colours; body colours; miniature; distemper; fresco; oil; encaustic paintings; enamel; mosaic work. Writing; materials for writing; pens; inks; use of coloured inks for denoting numbers; polygraph; telegraph; geometrical instruments; rulers; compasses; flexible rulers; squares; triangular compasses; parallel rulers; Marquois's scales; pantograph; proportional compasses; sector. Measurement of angles; theodolites; quadrants; dividing-engine; vernier; levelling; sines of angles; Gunter's scale; Nicholson's circle; dendrometer; arithmetical machines; standard measures; quotation from Laplace; new measures; decimal divisions; length of the pendulum and of the meridian of the earth; measures of time; objections; comparison of measures; instruments for measuring; micrometrical scales; log-lines."

This represents an extensive area to cover in a lecture of one hour.

When Newton, by means of a prism,

he showed that sunlight is made up of light varying in tint from red, through orange, yellow, green, and blue, to violet, and that by recombining all these kinds of light, or certain of them selected in an indefinite number of ways, white light could be produced. Subsequently Sir Wm. Herschel showed that rays less refrangible than the red were to be found among the solar radiation; and other rays more refrangible than the violet, but, like the ultra-red rays, incapable of exciting vision, were found by Ritter and Wollaston. In speaking of Newton's experiments, in his thirty-seventh lecture, Young says:—

It is certain that the perfect sensations of yellow and of blue are produced respectively by mixtures of red and green and of green and violet light, and there is reason to suspect that those sensations are always compounded of the separate sensations combined; at least, this supposition simplifies the theory of colours. It may, therefore, be adopted with advantage, until it be found inconsistent with any of the phenomena; and we may consider white light as composed of a mixture of red, green, and violet only, ... with respect to the quantity or intensity of the sensations produced.

It should be noticed that, in the above quotation, Young speaks only of the sensations produced. Objectively considered, sunlight consists of an infinite number of differently coloured lights comprising nearly all the shades from one end of the spectrum to the other, though white light may have a much simpler constitution, and may, for example, consist simply of a mixture of homogeneous red, green, and violet lights, or of homogeneous yellow and blue lights, properly selected. But considered subjectively, Young implies that the eye perceives three, and only three, distinct colour-sensations, corresponding to pure red, green, and violet; that when these three sensations are excited in a certain proportion, the complex sensation is that of white light; but if the relative intensities of the separate sensations differ from these ratios, the perception is that of some colour. To exhibit the effects of mixing light of different colours, Young painted differently coloured sectors on circles of cardboard, and then made the discs rotate rapidly about their centres, when the effect was the same as though the lights emitted by the sectors were mixed in proportion to the breadth of the sectors. This contrivance had been previously employed by Newton, and will be again referred to in connection with another memoir. The results of these experiments were embodied by Young in a diagram of colour, consisting of an equilateral triangle, in which the colours red, green, and violet, corresponding to the simple sensations, were placed at the angles, while those produced by mixing the primary colours in any proportions, were to be found within the triangle or along its sides; the rule being that the colour formed by the admixture of the primary colours in any proportions, was to be found at the centre of gravity of three heavy particles placed at the angular points of the triangle, with their masses proportioned to the corresponding amounts of light. Thus the colours produced by the admixture of red and green only, in different proportions, were placed along one side of the triangle, these colours corresponding to various tints of scarlet, orange, yellow, and yellowish green; another side contained the mixtures of green and violet representing the various shades of bluish green and blue; and the third side comprised the admixtures of red and violet constituting crimsons and purples. The interior of the triangle contained the colours corresponding to the mixture of all three primary sensations, the centre being neutral grey, which is a pure white faintly illuminated. If white light of a certain degree of intensity fall on white paper, the paper appears white, but if a stronger light fall on another portion of the same sheet, that which is less strongly illuminated appears grey by contrast. Shadows thrown on white paper may possess any degree of intensity, corresponding to varying shades of neutral grey, up to absolute blackness, which corresponds to a total absence of light. Thus considered, chromatically black and white are the same, differing only in the amount of light they reflect. A piece of white paper in moonlight is darker than black cloth in full sunlight.

It must be remembered that Young's diagram of colours corresponds to the admixture of coloured lights, not of colouring materials or pigments. The admixture of blue and yellow lights in proper proportions may make white or pink, but never green. The admixture of blue and yellow pigments makes a green, because the blue absorbs nearly all the light except green, blue, and a little violet, while the yellow absorbs all except orange, yellow, and green. The green light is the only light common to the two, and therefore the only light which escapes absorption when the pigments are mixed. Another point already noticed must also be carefully borne in mind. Young was quite aware that, physically, there are an infinite number of different kinds of light differing continuously in wave-length from the ultra-red to the ultra-violet, though colour can hardly be regarded as an attribute of the light considered objectively. The question of colour is essentially one of perception—a physiological, not a physical, question—and it is only in this sense that Young maintained the doctrine of three primary colours. In his paper on the production of colours, read before the Royal Society on July 1, 1802, he speaks of "the proportions of the sympathetic fibres of the retina," corresponding to these primary colour-sensations. According to this doctrine, white light would always be produced when the three sensations were affected in certain proportions, whether the exciting cause were simply two kinds of homogeneous light, corresponding to two pure tones in music, or an infinite number of different kinds, as in sunlight; and a particular yellow sensation might be excited by homogeneous yellow light from one part of the spectrum, or by an infinite number of rays of different wave-lengths, corresponding to various shades of red, orange, yellow, and green. Subjectively, the colours would be the same; objectively, the light producing them would differ exceedingly.

But Young's greatest service to science was his application of the principle of interference—of which he had already made good use in the theory of sound—to the phenomena of light. The results of these researches were presented to the Royal Society, and two of the papers were selected as Bakerian lectures in 1801 and 1803 respectively. Unfavourable criticisms of these papers, which appeared in the Edinburgh Review, and were said to have been written by Mr. (afterwards Lord) Brougham, seem to have caused their contents to be neglected by English men of science for many years; and it was to Arago and Fresnel that we are indebted for recalling public attention to them. The undulatory theory of light, which maintains that light consists of waves transmitted through an ether, which pervades all space and all matter, owes its origin to Hooke and Huyghens. Huyghens showed that this theory explained, in a very beautiful manner, the laws of reflection and of refraction, if it be allowed that light travels more slowly the denser the medium. According to the celebrated principle of Huyghens, every point in the front of a wave at any instant becomes a centre of disturbance, from which a secondary wave is propagated. The fronts of these secondary waves all lie on a surface, which becomes the new surface of the primary wave. When light enters a denser medium obliquely, the secondary waves which are propagated within the denser medium extend to a less distance than those propagated in the rarer medium, and thus the front of the primary wave becomes bent at the point where it meets the common surface. Huyghens explained, not only the laws of ordinary refraction in this manner, but, by supposing the secondary waves to form spheroids instead of spheres, he obtained the laws of refraction of the extraordinary ray in Iceland-spar. He did not, however, succeed in explaining why light should not diverge laterally instead of proceeding in straight lines. Newton supported the theory that light consists of particles or corpuscles projected in straight lines from the luminous body, and sometimes transmitted, sometimes reflected, when incident on a transparent medium of different density. To account for the particle being sometimes transmitted and sometimes reflected, Newton had recourse to the hypothesis of "fits of easy transmission and of easy reflection," and, to account for the fits themselves, he supposed the existence of an ether, the vibrations of which affected the particles. The laws of reflection were readily explained, being the same as for a perfectly elastic ball; the laws of refraction admitted of very simple explanation, by supposing that the particles of the denser medium exert a greater attraction on the particles of light than those of the rarer medium, but that this attraction acts only through very short distances, so that when the light-corpuscle is at a sensible distance from the surface, it is attracted equally all round, and moves as though there were no force acting upon it. As a consequence of this hypothesis, it follows that the velocity of light must be greater the denser the medium, while the undulatory theory leads to precisely the opposite result. When Foucault directly measured the velocity of light both in air and water, and found it less in the denser medium, the result was fatal to the corpuscular theory.

Dr. Young called attention to another crucial test between the two theories. When a piece of plate-glass is pressed against a slightly convex lens, or a watch-glass, a series of coloured rings is formed by reflected light, with a black spot in the centre. This was accounted for by Newton by supposing that the light which was reflected in any ring was in a fit of easy transmission (from glass to air) when it reached the first surface of the film of air, and in a fit of easy reflection when it reached the second surface. By measuring the thickness of a film of air corresponding to the first ring of any particular colour, the length of path corresponding to the interval between two fits for that particular kind of light could be determined. When water instead of air is placed between the glasses, according to the corpuscular theory the rings should expand; but according to the undulatory theory they should contract; for the wave-length corresponds to the distance between successive fits of the same kind on the corpuscular hypothesis. On trying the experiment, the rings were seen to contract. This result seemed to favour the undulatory theory; but the objection urged by Newton that rays of light do not bend round obstacles, like waves of sound, still held its ground. This objection Young completely demolished by his principle of interference. He showed that when light passes through an aperture in a screen, whatever the shape of the aperture, provided its width is large in comparison with the length of a wave of light (one fifty-thousandth of an inch), no sensible amount of light will reach any point not directly in front of the aperture; for if any point be taken to the right or left, the disturbances reaching that point from different points of the aperture will neutralize one another by interference, and thus no light will be appreciable. When the breadth of the aperture is only a small multiple of a wave-length, then there will be some points outside the direct beam at which the disturbances from different points of the aperture will not completely destroy one another, and others at which they will destroy one another; and these points will be different for light of different wave-lengths. In this way Young not only explained the rectilinear propagation of light, but accounted for the coloured bands formed when light diverges from a point through a very narrow aperture. In a similar way he accounted for the hyperbolic bands of colour observed by Grimaldi within the shadow of a square near its corners. With a strip of card one-thirtieth of an inch in width, Young obtained bands of colour within the shadow which completely disappeared when the light was cut off from either side of the strip of card, showing that they were produced by interference of the two portions of light which had passed, one to the right, the other to the left, of the strip of card. Professor Stokes has succeeded in showing a bright spot at the centre of the shadow of a circular disc of the size of a sovereign. The narrow bands of colour formed near the edge of the shadow of any object, which Newton supposed to be due to the "inflection" of the light by the attraction of the object, Young showed to be independent of the material or thickness of the edge, and completely accounted for them by the principle of interference. Newton's rings were explained with equal facility. They were due to the interference of light reflected from the first and second surfaces of the film of air or water between the glasses. The black spot at the centre of the reflected rings was due to the difference between reflection taking place from the surface of a denser or a rarer medium, half an undulation being lost when the reflection takes place in glass at the surface of air. If a little grease or water be placed between two pieces of glass which are nearly in contact, but the space between be not filled with the water or grease, but contain air in some parts, and water or grease in others, a series of rings will be seen by transmitted light, which have been called "the colours of mixed plates." Young showed that these colours could be accounted for by interference between the light that had passed through the air and that which had passed through the water, and explained the fact that, to obtain the same colour, the distance between the plates must be much greater than in the case of Newton's rings.

The bands of colour produced by the interference of light proceeding from a point and passing on each side of a narrow strip of card, have already been referred to. The bands are broader the narrower the strip of card. A fine hair gives very broad bands. When a number of hairs cross one another in all directions, these bands form circular rings of colour. If the width of the hairs be very variable, the rings formed will be of different sizes and overlapping one another, no distinct series will be visible; but when the hairs are of nearly the same diameter, a series of well-defined circles of colour, resembling Newton's rings, will be seen, and if the diameter of a particular ring be measured, the breadth of the hairs can be inferred. Young practically employed this method for measuring the diameter of the fibres of different qualities of wool in order to determine their commercial value. The instrument employed he called the eriometer. It consisted of a plate of brass pierced with a round hole about one-thirtieth of an inch in diameter in the centre, and around this a small circle, about one-third of an inch in diameter, of very fine holes. The plate was placed in front of a lamp, and the specimen of wool was held on wires at such a distance in front of the brass plate that the first green ring appeared to coincide with the circle of small holes. The eye was placed behind the lock of wool, and the distance to which the wool had to be removed in front of the brass plate in order that the first green ring might exactly coincide with the small circle of fine holes, was proportional to the breadth of the fibres. The same effect is produced if fine particles, such as lycopodium powder, or blood-corpuscles, scattered on a piece of glass, be substituted for the lock of wool, and Young employed the instrument in order to determine the diameter of blood-corpuscles. He determined the constant of his apparatus by comparison with some of Dr. Wollaston's micrometric observations. The coloured halos sometimes seen around the sun Young referred to the existence of small drops of water of nearly uniform diameter, and calculated the necessary diameter for halos of different angular magnitudes.

The same principle of interference afforded explanation of the colours of striated surfaces, such as mother-of-pearl, which vary with the direction in which they are seen. Viewed at one angle light of a particular colour reflected from different ridges will be in a condition to interfere, and this colour will be absent from the reflected light. At a different inclination, the light reaching the eye from all the ridges (within a certain angle) will be in precisely the same phase, and only then will light of that colour be reflected in its full intensity. With a micrometer scale engraved on glass by Coventry, and containing five hundred lines to the inch, Young obtained interference spectra. Modern gratings, with several thousand lines to the inch, afford the purest spectra that can be obtained, and enable the wave-length of any particular kind of light to be measured with the greatest accuracy.

Young's dislike of mathematical analysis prevented him from applying exact calculation to the interference phenomena which he observed, such as subsequently enabled Fresnel to overcome the prejudice of the French Academy and to establish the principle on an incontrovertible footing. Young's papers attracted very little attention, and Fresnel made for himself many of Young's earlier discoveries, but at once gave Young the full credit of the work when his priority was pointed out. The phenomena of polarization, however, still remained unexplained. Both Young and Fresnel had regarded the vibrations of light as similar to those of sound, and taking place in the direction in which the wave is propagated. The fact that light which had passed through a crystal of Iceland-spar, was differently affected by a second crystal, according to the direction of that crystal with respect to the former, showed that light which had been so transmitted was not like common light, symmetrical in all azimuths, but had acquired sides or poles. Such want of symmetry could not be accounted for on the hypothesis that the vibrations of light took place at right angles to the wave-front, that is, in the direction of propagation of the light. The polarization of light by reflection was discovered by Malus, in 1809. In a letter written to Arago, in 1817, Young hinted at the possibility of the existence of a component vibration at right angles to the direction of propagation, in light which had passed through Iceland-spar. In the following year Fresnel arrived independently at the hypothesis of transverse vibrations, not as constituting a small component of polarized light, but as representing completely the mode of vibration of all light, and in the hands of Fresnel this hypothesis of transverse vibrations led to a theory of polarization and double refraction both in uniaxal and biaxal crystals which, though it can hardly be regarded as complete from a mechanical point of view, is nevertheless one of the most beautiful and successful applications of mathematics to physics that has ever been made. To Young, however, belongs the credit of suggesting that the spheroidal form of the waves in Iceland-spar might be accounted for by supposing the elasticity different in the direction of the optic axis and at right angles to that direction; and he illustrated his view by reference to certain experiments of Chladni, in which it had been shown that the velocity of sound in the wood of the Scotch fir is different along, and perpendicular to, the fibre in the ratio of 5 to 4. Young was also the first to explain the colours exhibited by thin plates of crystals in polarized light, discovered by Arago in 1811, by the interference of the ordinary and extraordinary rays, and Fresnel afterwards completed Young's explanation in 1822.

It is for his contributions to the undulatory theory of light that Young will be most honourably remembered. Hooke, in 1664, referred to light as a "quick, short, vibrating motion;" Huyghens's "TraitÉ de la LumiÈre" was published in 1690. From that time the undulatory theory lost ground, until it was revived by Young and Fresnel. It soon after received great support from the establishment, by Joule and others, of the mechanical theory of heat. One remark of Young's respecting the ether opens up a question which has attracted much attention of late years. In a letter addressed to the Secretary of the Royal Society, and read January 16, 1800, he says:—

That a medium, resembling in many properties that which has been denominated ether, does really exist, is undeniably proved by the phenomena of electricity; and the arguments against the existence of such an ether throughout the universe have been pretty sufficiently answered by Euler. The rapid transmission of the electrical shock shows that the electric medium is possessed of an elasticity as great as is necessary to be supposed for the propagation of light. Whether the electric ether is to be considered as the same with the luminous ether—if such a fluid exists—may perhaps at some future time be discovered by experiment.

Besides his contributions to optics, Young made distinct advances in connection with elasticity, and with surface-tension, or "capillarity." It is said that Leonardo da Vinci was the first to notice the ascent of liquids in fine tubes by so-called capillary attraction. This, however, is only one of a series of phenomena now very generally recognized, and all of which are referable to the same action. The hanging of a drop from the neck of a phial; the pressure of air required to inflate a soap-bubble; the flotation of a greasy needle on the surface of water; the manner in which some insects rest on water, by depressing the surface, without wetting their legs; the possibility of filling a tumbler with water until the surface stands above the edge of the glass; the nearly spherical form of rain-drops and of small drops of mercury, even when they are resting on a table,—are all examples of the effect of surface-tension. These phenomena have recently been studied very carefully by Quincke and Plateau, and they have been explained in accordance with the principle of energy by Gauss. Hawksbee, however, was the first to notice that the rise of a liquid in a fine tube did not depend on the thickness of the walls of the tube, and he therefore inferred that, if the phenomena were due to the attraction of the glass for the liquid, it could only be the superficial layers which produced any effect. This was in 1709. Segner, in 1751, introduced the notion of a surface-tension; and, according to his view, the surface of a liquid must be considered as similar to a thin layer of stretched indiarubber, except that the tension is always the same at the surface bounding the same media. This idea of surface-tension was taken up by Young, who showed that it afforded explanation of all the known phenomena of "capillarity," when combined with the fact, which he was himself the first to observe, that the angle of contact of the same liquid-surface with the same solid is constant. This angle he called the "appropriate angle." But Young went further, and attempted to explain the existence of surface-tension itself by supposing that the particles of a liquid not only exert an attractive force on one another, which is constant, but also a repulsive force which increases very rapidly when the distance between them is made very small. His views on this subject were embodied in a paper on the cohesion of liquids, read before the Royal Society in 1804. He afterwards wrote an article on the same subject for the supplement of the "EncyclopÆdia Britannica."

The changes which solids undergo under the action of external force, and their tendency to recover their natural forms, were studied by Hooke and Gravesande. The experimental fact that, for small changes of form, the extension of a rod or string is proportional to the tension to which it is exposed, is known as Hooke's law. The compression and extension of the fibres of a bent beam were noticed by James Bernoulli, in 1630, by Duhamel and others. The bending of beams was also studied by Coulomb and Robison, but Young appears to have been almost the first to apply the theory of elasticity to the statics of structures. In a letter to the Secretary of the Admiralty, written in 1811, in reply to an invitation to report on Mr. Steppings's improvements in naval architecture, Young claimed that he was the only person who had published "any attempts to improve the theory of carpentry." It may be here mentioned that Young accepted the invitation of the Admiralty, and sent in a very exhaustive report, which their Lordships regarded as "too learned" to be of great practical value. Young's contributions to this subject will be chiefly remembered in connection with his "modulus of elasticity." This he originally defined as follows:—

"The modulus of the elasticity of any substance is a column of the same substance capable of producing a pressure on its base which is to the weight causing a certain degree of compression as the length of the substance is to the diminution of its length."

It is not usual now to express Young's modulus of elasticity in terms of a length of the substance considered. As now usually defined, Young's modulus of elasticity is the force which would stretch a rod or string to double its natural length if Hooke's law were true for so great an extension.

So much of Dr. Young's scientific work has been mentioned here because it was during his early years of professional practice that his most original scientific work was accomplished. As already stated, after two years' tenure of the Natural Philosophy chair at the Royal Institution, Young resigned it because his friends were of opinion that its tenure militated against his prospects as a physician. In the summer of 1802 he escorted the great-nephews of the Duke of Richmond to Rouen, and took the opportunity of visiting Paris. In March, 1803, he took his degree of M.B. at Cambridge, and on June 14, 1804, he married Eliza, second daughter of J. P. Maxwell, Esq., whose country seat was near Farnborough. For sixteen years after his marriage, Young resided at Worthing during the summer, where he made a very respectable practice, returning to London in October or November. In January, 1811, he was elected one of the physicians of St. George's Hospital, which appointment he retained for the rest of his life. In this capacity his practice was considerably in advance of the times, for he regarded medicine as a science rather than an empirical art, and his careful methods of induction demanded an amount of attention which medical students, who preferred the more rough-and-ready methods then in vogue, were slow to give. The apothecary of the hospital stated that more of Dr. Young's patients went away cured than of those who were subjected to the more fashionable treatment; but his private practice, notwithstanding the sacrifices he had made, never became very valuable.

In 1816 Young was appointed Secretary to a Commission for determining the length of the second's pendulum. The reports of this Commission were drawn up by him, though the experimental work was carried out by Captain Kater. The result of the work was embodied in an Act of Parliament, introduced by Sir George Clerk, in 1824, which provided that if the standard yard should be lost it should "be restored to the same length," by making it bear to the length of the second's pendulum at sea-level in London, the ratio of 36 to 39·1393; but before the standards were destroyed, in 1835, so many sources of possible error were discovered in the reduction of pendulum observations, that the Commission appointed to restore the standards recommended that a material standard yard should be constructed, together with a number of copies, so that, in the event of the standard being again destroyed, it might be restored by comparison with its copies. In 1818 Young was appointed Superintendent of the Nautical Almanac and Secretary of the Board of Longitude. When this Board was dissolved in 1828, its functions were assumed by the Admiralty, and Young, Faraday, and Colonel Sabine were appointed a Scientific Committee of Reference to advise the Admiralty in all matters in which their assistance might be required. The income from these Government appointments rendered Young more independent of his practice, and he became less careful to publish his scientific papers anonymously. In 1820 he left Worthing and gave up his practice there. The following year, in company with Mrs. Young, he took a tour through France, Switzerland, and Italy, and at Paris attended a meeting of the Institute, where he met Arago, who had called on him in Worthing, in 1816. At the same time he made the acquaintance of Laplace, Cuvier, Humboldt, and others. In 1824 he visited Spa, and took a tour through Holland. In the same year Young was appointed Inspector of Calculations and Medical Referee to the Palladium Insurance Company. This caused him to turn his attention to the subject of life assurance and bills of mortality. In 1825, as Foreign Secretary of the Royal Society, he had the satisfaction of forwarding to Fresnel the Rumford Medal in acknowledgment of his researches on polarized light. Fresnel died, in his fortieth year, a few days after receiving the medal.

Dr. Young died on May 10, 1829, in the fifty-sixth year of his age, his excessive mental exertions in early life having apparently led to a premature old age. He was buried in the parish church of Farnborough, and a medallion by Sir Francis Chantrey was erected to his memory in Westminster Abbey.

But, though Young was essentially a scientific man, his accomplishments were all but universal, and any memoir of him would be very incomplete without some sketch of his researches in Egyptian hieroglyphics. His classical training, his extensive knowledge of European and Eastern languages, and his neat handwriting and drawing, have already been referred to. To these attainments must be added his scientific method and power of careful and systematic observation, and it will be seen that few persons could come to the task of deciphering an unknown language with a better chance of success than Dr. Young.

The Rosetta Stone was found by the French while excavating at Fort St. Pierre, near Rosetta, in 1799, and was brought to England in 1802. The stone bore an inscription in three different kinds of character—the Hieroglyphic, the Enchorial or Demotic, and the ordinary Greek. Young's attention was first called to the Egyptian characters by a manuscript which was submitted to him in 1814. He then obtained copies of the inscriptions on the Rosetta Stone and subjected them to a careful analysis. The latter part of the Greek inscription was very much injured, but was restored by the conjectures of Porson and Heyne, and read as follows:—"What is here decreed shall be inscribed on a block of hard stone, in sacred, in enchorial, and in Greek characters, and placed in each temple, of the first, second, and third gods."

This indicated that the three inscriptions contained the same decree, but, unfortunately, the beginnings of the first and second inscriptions were lost, so that there were no very definitely fixed points to start upon. The words "Alexander" and "Alexandria," however, occurred in the Greek, and these words, being so much alike, might be recognized in each of the other inscriptions. The word "Ptolemy" appeared eleven times in the Greek inscription, and there was a word which, from its length and position, seemed to correspond to it, which, however, appeared fourteen times in the hieroglyphic inscription. This word, whenever it appeared in the hieroglyphics, was surrounded by a ring forming what Champollion called a cartouche, which was always employed to denote the names of royal persons. These words were identified by Baron Sylvestre de Sacy and the Swedish scholar Akerblad. Young appears to have started with the idea, then generally current, that hieroglyphic symbols were purely ideographic, each sign representing a word. His knowledge of Chinese, however, led him to modify this view. In that language native words are represented by single symbols, but, when it is necessary to write a foreign word, a group of word-symbols is employed, each of which then assumes a phonetic character of the same value as the initial letter of the word which it represents. The phonetic value of these signs is indicated in Chinese by a line at the side, or by enclosing them in a square. Young supposed that the ring surrounding the royal names in the hieroglyphic inscription had the same value as the phonetic mark in Chinese, and from the symbols in the name of Ptolemy he commenced to construct a hieroglyphic alphabet. He made an error, however, in supposing that some of the symbols might be syllabic instead of alphabetic. It is true that in the older inscriptions single signs have sometimes a syllabic value, and sometimes are used ideographically, while in other cases a single sign representing the whole word is employed in conjunction with the alphabetic signs, probably to distinguish the word from others spelt in the same way, but in inscriptions of so late a date as the Rosetta Stone, the symbols were purely alphabetic. Another important step made by Young was the discovery of the use of homophones, or different symbols to represent the same letter. Young's work was closely followed up by Champollion, and afterwards by Lepsius, Birsch, and others. The greater part of his researches he never published, though he made careful examinations of several funeral rolls and other documents.

It would occupy too much space to give an adequate account of Young's researches in this subject; some portion of his work he published in a popular form in the article "Egypt," in the supplement of the "EncyclopÆdia Britannica," to which supplement he contributed about seventy articles on widely different subjects. Perhaps it is not too much to say that to Young we owe the foundation of all we now know of hieroglyphics and the Egyptian history which has been learned from them; and the obelisk on the Thames Embankment should call to mind the memory of no one more prominently than that of Thomas Young.