George Adams

As the advantages which are obtained from any instrument are considerably increased, if it be used by a person who is master of its properties, attentive to its adjustments, and habituated by practice to the minutiÆ of management, it is the design of this chapter to point out those circumstances which more peculiarly require the attention of the observer, and to give such plain directions, as may enable him to examine any object with ease; to shew how he may place it in the best point of view, and if necessary, prepare it for observation.

A small degree of diligence will render the observer master of every necessary rule, and a little practice will make them familiar and habitual: the pains he takes to acquire these habits will be rewarded by an increasing attachment to his instrument, and the wonders it displays. Let him only persevere till he has overcome the natural indolence that opposes the advancement of every kind of knowledge, and he will most assuredly find himself very amply recompensed, by the gratification arising from the acquisition of a science that has the unlimited treasures of INFINITE WISDOM for the object of its researches: and his mind being strengthened by the victory it has gained, will be more keen in perceiving, and more patient in the investigation of truth.

It has long been a complaint,[39] that many of those who purchase microscopes are so little acquainted with their general and extensive usefulness, and so much at a loss for objects to examine by them, that after diverting themselves and their friends some few times with what they find in the sliders, which generally accompany the instrument, or perhaps two or three common objects, the microscope is laid aside as of little further value: whereas no instrument has yet appeared in the world capable of affording so constant, various, and satisfactory an entertainment to the mind. This complaint will, I hope, be obviated by these Essays, in which I have endeavoured to make the use of the microscope easy, point out an immense variety of objects, and direct the observer how to prepare them for examination.

[39] Baker’s Microscope made Easy, p. 51.

The subject treated of in this chapter naturally divides itself into three heads: the first describes the necessary preparation and adjustment of the microscope; the second treats of the proper quantity of the light, and the best method of adapting it to the objects under examination; and the third shews how to prepare and preserve the various objects, that their nature, organization, and texture, may be properly understood.

OF THE NECESSARY PREPARATION OF THE MICROSCOPE FOR OBSERVATION.

We have in the last chapter explained those particulars that constitute the difference of one microscope from another, and shewn the manner of using each instrument, and how the several parts are to be applied to it. We shall now proceed to give some general directions applicable to every microscope. The observer is therefore supposed to have made himself master of his instrument, and to know how to adapt the different parts of the apparatus to their proper places.

The first circumstance necessary to be examined into, is, whether the different glasses belonging to the microscope are perfectly clean or not; if they be not clean, they must be taken out and wiped with a piece of wash leather, taking care at the same time not to soil the surface of the glass with the fingers: in replacing the glasses, you must also be careful not to lay them in an oblique situation, to place the convex sides as before, and if one glass be taken out, wiped, and replaced before the next, it may prevent the misplacing of them by an unskilful hand.

The object should be brought as near the center of the field of view as possible, for there only will it be exhibited in the greatest perfection.

The eye should be moved up and down from the eye-glass of a compound microscope, till you find that situation where the largest field, and most distinct view of the object is obtained; and as the sight differs very much in different persons, and even in the same person, we frequently find each eye to have a different sight from the other, particularly in those called myopes, or short-sighted, every one ought to adjust the microscope to his own eye, and not depend upon the situation in which it was placed by another.

Care must be taken not to let the breath fall upon the eye-glass, nor to hold that part of the body of the microscope where the glasses are placed with a warm hand, because the damp that is expelled from the metal by the heat will be attracted and condensed by the glasses, and obstruct the sight of the object.

The observer should always begin with a small magnifying power; with this he will gain an accurate idea of the situation and connection of the whole, and will therefore be less liable to form any erroneous opinion, when the parts are viewed separately by a deeper lens. By a shallow magnifier he will also discover those parts which merit a further investigation. Objects that are transparent will bear a much greater magnifying power than those that are opake.

Every object should, if possible, be examined first in that position which is most natural to it: if this circumstance be neglected, very inadequate ideas of the structure of the whole, as well as of the connection and use of the parts, will be formed. If it be a living animal, care must be taken not to squeeze, hurt, or discompose it.

There is a great difference between merely viewing an object by the microscope, and investigating its nature: in the first, we only consider the magnified representation thereof; in the second, we endeavour to analyse and discover its nature and relation to other objects. In the first case, we receive the impression of an image formed by the action of the glasses; in the second, we form our judgment by investigating this image. It is easy to view the image which is offered to the eye, but not so easy to form a judgment of the things that are seen; an extensive knowledge of the subject, great patience, and many experiments, will be found necessary for this purpose: for there are many circumstances where the images seen may be very similar, though originating from substances totally different; it is here the penetration of the observer will be exercised, to discover the difference, and avoid error.[40]

Hence Mr. Baker cautions us against forming too suddenly an opinion of any microscopic object, and not to draw our inferences till after repeated experiments and examinations of the objects, in all lights and various positions; to pass no judgment upon things extended by force, or contracted by dryness, or in any manner out of a natural state, without making suitable allowances.

The true colour of objects cannot be properly determined when viewed through the deepest magnifiers; for, as the pores and interstices of an object are enlarged, according to the magnifying power of the glasses made use of, the component parts of its substance will appear separated many thousand times farther asunder than they do to the naked eye; it is, therefore, very probable, that the reflection of the light from these particles will be very different, and exhibit different colours.

Some consideration is also necessary in forming a judgment of the motion of living creatures, or even of fluids, when seen through the microscope; for as the moving body, and the space wherein it moves, are magnified, the motion will also be increased.

If an object be so opake as not to suffer any light to pass through it, as much as possible must be thrown on its upper surface, by that part of the apparatus which is peculiarly adapted to opake objects. As the apertures of deep magnifiers are but small, and consequently admit but little light, they are not proper for the examination of opake objects: this, however, naturally leads us to our second head.

OF THE MANAGEMENT OF THE LIGHT.

The pleasure arising from a just view of a microscopic object, the distinctness of vision, &c. depend on a due management of the light, and adapting the quantity of it to the nature of the object, and the focus of the magnifier; therefore, an object should always be viewed in various degrees of light. It is difficult to distinguish in some objects between a prominency and a depression, between a shadow and a black stain; and in colour, between a reflection and a whiteness; a truth which the reader will find fully exemplified in the examination of the eye of the libellula, and other flies, which will be found to appear exceedingly different in one position of the light from what they do in another.

The brightness of an object depends on the quantity of light; the distinctness of vision, on regulating the quantity to the object; for some will be lost and drowned, as it were, in a quantity of light that is scarce sufficient to render another visible, as a different portion of light under the same apparatus will often exhibit in perfection, or totally conceal an object in the substance to be examined. This is more particularly the case with the animalculÆ infusoriÆ, whose thin and transparent form blend as it were with the water in which they swim; the degree of light must therefore be suited to the object, which, if dark, will be seen best in a strong and full light, but if very transparent, it should be examined in a fainter.

A strong light may be thrown on an object various ways: first, by means of the sun and a convex lens; for this purpose, place the microscope about three feet from a southern window; take a deep convex lens, that is mounted in a semicircle and fixed on a stand, so that its position may be easily varied; place this lens between the object and the window, so that it may collect a considerable number of the solar rays, and refract them on the object, or the mirror of the microscope. If the light thus collected from the sun be too powerful, it may be tempered by placing a piece of oil paper, or a glass lightly greyed, between the object and the lens: by these means, a convenient degree of light may be obtained, and diffused in an equal manner over the whole surface of an object, a circumstance that should be particularly attended to; for if the light be thrown in an irregular manner, that is, larger portions of it on some parts than on others, it will not be distinctly exhibited.

Where the solar light is preferred, it will be found very convenient to darken the room, and to reflect the rays of the sun on the above mentioned lens, by means of the mirror of a solar microscope fitted to the window-shutter; for, by this apparatus, the observer will be enabled to preserve the light on his object, notwithstanding the motion of the sun.

Cutting off the adventitious light as much as possible, by darkening the room where you are using the microscope, and admitting the light only through a hole in the window-shutter, or at most, keeping one window only open, will also be found very conducive towards producing a distinct view of the object.

As the motion of the sun, and the variable state of our atmosphere, render solar observations both tedious and inconvenient, it will be proper for the observer to be furnished with a large tin lanthorn, made something like the common magic lanthorn, fit to contain one of Argand’s lamps.[41] The lanthorn should have an aperture in front, that may be moved up and down, and capable of holding a lens; by this a pleasing uniform dense light may be easily procured. The lamp should move on a rod, that it may be readily elevated or depressed. The lanthorn may be used for many other purposes, as for viewing of pictures, exhibiting microscopic objects on a screen, &c.

Many transparent objects are seen best in a weak light; among these we may place the prepared eyes of flies and animalculÆ in fluids; the quantity of light from a lamp or candle may be lessened by removing the microscope to a greater distance from them, or it may be more effectually lessened by cutting off a part of the cone of rays that fall on the object, either by placing the cone, as already described with the apparatus to different microscopes, under the stage, or by forming circular apertures of black paper of different sizes, and placing either a large or small one on the reflecting mirror, as occasion may require.

There is an oblique position of the mirrors, and consequently of the light, which is easily acquired by practice, but for which no general rule can be given, that will exhibit an object more beautifully and more distinctly than any other situation, shewing the surface, as well as those parts through which the light is transmitted.

A better view of most objects is obtained by a candle or lamp than by day-light; it is more easy to modify the former than the latter, and to throw it on the object with different degrees of density. From what has been said, the reader will have observed the importance of being able to examine the object in the greatest variety of positions and appearances, which cannot be effected with equal convenience by any microscope, but the improved lucernal.

OF THE PREPARATION OF OBJECTS FOR THE MICROSCOPE.

In the preparation of objects, no man was more successful or more indefatigable than Swammerdam. In minutely anatomizing, in patiently investigating, and in curiously exhibiting the minute wonders of the creation, he stands unrivalled, far exceeding all those that preceded, as well as those which have succeeded him. Deeply impressed and warmly animated by the amazing scenes that he continually discovered, his zeal in pursuit of truth was not to be abated by disappointment, or alarmed by difficulty; and he was never satisfied till he had attained a rational and clear idea of the organization of the object, whose structure he wished to explore; his “Book of Nature,” of which a translation was published by Dr. Hill, is a work of such vast extent of knowledge, and so excellent in execution, as to raise the highest admiration in even a superficial observer.

It is much to be regretted, that we are ignorant of the methods he employed in his investigations. To discover these, the great Boerhaave examined with a scrupulous attention all the letters and manuscripts of Swammerdam, and has communicated the result of his researches, which, though but small, may enable us to form some idea of his immense labours in the field of science.

For dissecting of small insects he had a brass table, which was made by that excellent artist, S. Musschenbroeck; to this table were affixed two brass arms, moveable at pleasure to any part of it. The upper portion of these arms was constructed so as to have a slow vertical motion, by which means the operator could readily alter their height, as he saw most convenient to his purpose; the office of one of these arms was to hold the minute bodies, and that of the other to apply the lens or microscope.

His microscopes or lenses were of various foci, diameters, and sizes, from the least to the greatest, and the best that could be procured in regard to the exactness of the workmanship, and transparency of the substance. His mode was, to begin his observations with the smallest magnifiers, and from thence proceed by degrees to the greatest. Formed by nature, and habituated by experience, he was so incomparably dexterous in the management of these instruments, that he made every observation subservient to the next, and all tend to confirm each other, and complete the description.

His chief art seems to have been in constructing very fine scissars, and giving them an extreme sharpness: these he made use of to cut very minute objects, because they dissected them equally; whereas knives and lancets, let them be ever so fine and sharp, are apt to disorder delicate substances, as in going through them, they generally draw after and displace some of the filaments. His knives, lancets and styles, were so very fine, that he could not see to sharpen them without the assistance of a magnifying glass; but with them he could dissect the intestines of bees with the same accuracy and distinctness that the most celebrated anatomist does those of large animals. He was particularly expert in the management of small glass tubes, which were no thicker than a bristle, and drawn to a very fine point at one end, but thicker at the other. These he made use of to shew and blow up the smallest vessels discovered by the microscope, to trace, distinguish, and separate their courses and communications, or to inject them with very subtil coloured liquors.

He used to suffocate the insects in spirit of wine, in water, or spirit of turpentine, and likewise preserved them for some time in these liquids; by which means he kept the parts from putrefaction, and consequently from collapsing and mixing together; and added to them besides such strength and firmness, as rendered the dissections more easy and agreeable. When he had divided transversely with his fine scissars the little creature he intended to examine, and had carefully noted every thing that appeared without further dissection, he then proceeded to extract the viscera in a very cautious and deliberate manner, with other instruments of great fineness; first taking care to wash away and separate with very fine pencils, the fat with which insects are very plentifully supplied, and which always prejudices the internal parts before it can be extracted. This operation is best performed upon insects while in the nympha state.

Sometimes he put into water the delicate viscera of the insects he had suffocated; and then shaking them gently he procured himself an opportunity of examining them, especially the air vessels, which by these means he could separate from all the other parts whole and intire, to the great admiration of all those who beheld them; as these vessels are not to be distinctly seen in any other manner, or indeed seen at all without damaging them, he often made use of water, injected by a syringe, to cleanse thoroughly the internal parts, then blew them up with air and dried them, and thus rendered them durable, and fit for examination at a proper opportunity. Sometimes he has examined with the greatest success, and made the most important discoveries in insects that he had preserved in balsam, and kept for years together in that condition. Again, he has frequently made punctures in other insects with a very fine needle, and after squeezing out all their moisture through the holes made in this manner, he filled them with air, by means of very slender glass tubes, then dried them in the shade, and last of all anointed them with oil of spike, in which a little rosin had been dissolved; by which process they retained their proper forms a long time. He had a singular secret, whereby he could so preserve the nerves of insects, that they used to continue as limber and perspicuous as ever they had been.

He used to make a small puncture or incision in the tail of worms, and after having gently and with great patience squeezed out all their humours, and great part of their viscera, he then injected them with wax, so as to give and continue to them all the appearance of healthy vigorous living creatures. He discovered that the fat of all insects was perfectly dissoluble in oil of turpentine; thus he was enabled to shew the viscera plainly; only after this dissolution he used to cleanse and wash them well and often in clean water. He frequently spent whole days in thus cleansing a single caterpillar of its fat, in order to discover the true construction of this insect’s heart. His singular sagacity in stripping off the skin of caterpillars that were upon the point of spinning their cones, deserves particular notice. This he effected by letting them drop by their threads into scalding water, and suddenly withdrawing them; for, by these means the epidermis peeled off very easily; and when this was done, he put them into distilled vinegar and spirit of wine, mixed together in equal portions, which, by giving a proper firmness to the parts, gave him an opportunity of separating them with very little trouble from the exuviÆ, or skins, without any danger to the parts; so that by this contrivance, the nymph could be shewn to be wrapped up in the caterpillar and the butterfly in the nymph. Those who look into the works of Swammerdam, will be abundantly gratified, whether they consider his astonishing labour and unremitted ardour in these pursuits, or his wonderful devotion and piety. On one hand, his genius urged him to examine the miracles of the great Creator in his natural, productions; whilst, on the other, the love of that same all-perfect Being rooted in his mind struggled hard to persuade him that God alone, and not the creatures, were worthy of his researches, love, and attention.

M. Lyonet always drowned first those insects he intended to anatomize, as by these means he was enabled to preserve both the softness and transparency of the parts. If the insect, &c. be very small, for instance one-tenth of an inch, or a little more in length, it should be dissected in water, on a glass which is a little concave; if, after a few days, there be any fear that the insect will putrefy, it should be placed in weak spirit of wine, instead of water. In order to fix the little creature, it must be suffered to dry, and then be fastened by a piece of soft wax; after which it may be again covered with water.

Larger objects require a different process; they should be placed in a small trough of thin wood; the bottom of a common chip box will answer very well, by surrounding the edge of it with soft wax, to keep in the water or spirit of wine. The insect is then to be opened, and if the parts be soft, like those of a caterpillar, they should be turned back and fixed to the trough by small pins; the pins are to be set by a pair of small nippers, the skin being stretched at the same instant by another pair of finer forceps; the insect must then be placed in water, and dissected therein, and after two or three days it should be covered with spirit of wine, which should be renewed occasionally; by these means the subject is preserved in perfection, and its parts may be gradually unfolded, without any other change being perceived than that the soft elastic parts become stiff and opake, and some others lose their colour.

M. Lyonet used the following instruments in his curious dissection of the caterpillar of the cossus. As small a pair of scissars as could be made, the arms long and fine; a small and sharp knife, the end brought to a point; a pair of forceps, the ends of which had been so adjusted, that they would easily lay hold of a spider’s thread or a grain of sand. But the most useful instruments were two fine steel needles, fixed in small wooden handles, about 2-³/₄ of an inch in length.

An observation of Dr. Hooke’s may be very useful if attended to, for fixing objects intended to be delineated by the microscope. He found no creature more troublesome to draw than the ant or pismire, not being able to get the body quite in a natural posture. If, when alive, its feet were fettered with wax or glue, it would so twist and twine its body, that it was impossible any way to get a good view of it; if it was killed, the body was so small, that the shape was often spoiled before it could be examined. It is the nature of many minute bodies, when their life is destroyed, for the parts to shrivel up immediately; this is very observable in many small plants, as well as in insects; the surface of these small bodies, if porous, being affected by almost every change of the air, and this is particularly the case with the ant. But if the little creature be dropped in well rectified spirit or wine, it is immediately killed; and when taken out, the spirit of wine evaporates, leaving the animal dry and in its natural posture, or at least so constituted, that you may easily place it with a pin in what posture you please.[42]

Having thus given a general account of the methods used by Swammerdam and Lyonet, in their examination and dissection of insects, we shall proceed to shew how to prepare several of their parts for the microscope, beginning with the WINGS. Many of these are so transparent and clear, as to require no previous preparation; but the under wings of those that are covered with elytra, or crustaceous cases, being constantly folded up when at rest, they must be unfolded before they can be examined by the microscope; for this purpose a considerable share of dexterity and some patience is necessary, for the natural spring of the wings is so strong, that they immediately fold themselves again, except they are carefully prevented.

One of the most curious and beautiful wings of this kind, is that of the FORFICULA AURICULARIA, or EARWIG, of which we have given a drawing, Plate XIV. Fig. 1, represents it considerably magnified, and Fig. 2, the same object of its natural size. When expanded, it is a tolerably large wing, yet folds up under a case not one-eighth part of its size. It is very difficult to unfold these wings, on account of their curious texture. They are best opened immediately after the insect is killed. Hold the earwig by the thorax, between the finger and thumb; then with a blunt-pointed pin endeavour gently to open the wing by spreading it over the fore-finger, gradually sliding at the same time the thumb over it. When fully expanded, separate it from the insect by a sharp knife, or a pair of scissars. The wing should be pressed for some time between the thumb and finger before it be removed; it may then be placed between two pieces of paper, and again pressed for at least an hour; after which it may be put between the talcs without any danger of folding up again.

The wings of the NOTONECTA, or BOAT-FLY, and other water insects, as well as most species of the grylli, require equal care and delicacy with that of the earwig to display them properly.

The wings of BUTTERFLIES and MOTHS are covered with very minute scales or feathers, that afford a beautiful object for the microscope; near the shoulder, the thorax, the middle of the wing, and the fringes of the wings, they are generally intermixed with hair. The scales of one part, also, often differ in shape from those of another; they may be first scraped off or loosened from the wing with a knife, and then brushed into a piece of paper with a camel’s hair pencil; the scales may be separated from the hairs with the assistance of a common magnifying glass.

The proboscis of insects, as of the CULEX or GNAT, the TABANUS or BREEZE-FLY, &c. requires much attention and considerable care to be dissected properly for the microscope; and many must be prepared before the observer desides upon the situation and shape of the parts; he will often also be able to unfold in one specimen some parts that he can scarce discover in another. It is well known that the COLLECTOR OF THE BEE forms a most beautiful object; a figure of it is given in plate XIII. Fig. 3, shews it greatly magnified, and Fig. 4, of the natural size. In it is displayed a most wonderful mechanism, admirably adapted to collect and extract the various sweets from flowers, &c. To prepare this, it should first be carefully washed with spirit of turpentine, by which means it will be freed from the unctuous and melliferous particles which usually adhere to it; when dry, it must be again washed with a camel’s hair pencil, to disengage and bring forward the small hairs which form one part of its microscopic beauty.

The case which encloses THE STING OF THE BEE, the wasp, and the hornet, are so hard, that it is very difficult to extract them without breaking or otherwise injuring them. It will be found, perhaps, the best way to soak the case, and the rest of the apparatus for some time in spirit of wine or turpentine, then lay it on a piece of clean paper, and with a blunt knife draw out the sting, holding the sheath by the nail of the finger, or by any blunt instrument; great care is requisite to preserve the feelers, which when cleaned add much to the beauty of the object.

The EYES OF THE LIBELLULA or DRAGON-FLY, and different flies, of the LOBSTER, &c. are first to be cleaned from the blood and other extraneous matter; they should then be soaked in water for some days, after which you may separate one or two skins from the eye, which, if they remain, render it too opake and confused; some care is, however requisite in this separation, otherwise the skin may be made too thin, so as not to enable you to form an accurate idea of its organization.

The EXUVIÆ or CAST-OFF OF SKINS of insects are in general very pleasing objects, and require but little preparation. If they be curled or bent up, keep them in a moist atmosphere for a few hours, and they will soon become so relaxed that you may extend them with ease to their natural positions. The steam of warm water answers the purpose very well.

The BEARD OF THE LEPAS ANATIFERA or BARNACLE is to be soaked in clean soft water, and frequently brushed, while wet, with a camel’s hair pencil; it may then be left to dry; after which it must be again brushed with a dry pencil, to disengage and separate the hairs, which are apt to adhere together. A picture of this object is represented in plate XIII. Fig. 1, magnified; Fig. 2, natural size.

To view the MUSCULAR FIBRES, take a very thin piece of dried flesh, lay it upon a slip of glass, and moisten it with warm water; when this is evaporated, the vessels will appear plain and more visible, and by repeated macerations the parts may be further disengaged.

To examine FAT, BRAINS, and other similar substances, we are advised by Dr. Hooke to render the surface smooth, by pressing it between two thin plates of flat glass, by which the substance will be made much thinner and more transparent; otherwise, the parts lying thick one upon the other, it appears confused and indistinct.

Some substances are, however, so organized, that if their peculiar form be altered, the parts we wish to discover are destroyed; such as nerves, tendons, muscular fibres, pith of wood, &c. Many of these are best to be examined while floating in some convenient transparent fluid. For instance, very few of the fibres of any of the muscles can be discovered when they are viewed in the open air; but if placed in water or oil, great part of their wonderful fabric may be discovered. If the thread of a ligament be viewed in this manner, it will be seen to consist of an indefinite number of smooth round threads lying close together.

Objects of an elastic nature should be pulled or stretched out while they are under the microscope, that the texture and nature of those parts, whose figure is altered by being thus pulled out, may be more fully discovered.

To examine BONES with the microscope. These should first be viewed as opake objects; afterwards, by procuring thin sections, they should be looked at as if transparent. The sections should be cut in all directions, and be well washed and cleaned; a degree of maceration will be useful in some cases. Or the bones may be put in a clear fire till they are red hot, and then taken out; by these means the bony cells will appear more conspicuous and visible, being freed from extraneous matter.

To examine the PORES OF THE SKIN. First, cut or pare off with a razor as thin a slice as possible of the upper skin; then cut a second from the same place; apply the last to the microscope.

The SCALES OF FISH should be soaked in water for a few days, and then be carefully rubbed, to clean them from the skin and dirt which may adhere to them.

To procure the scales of the eel, which are a great curiosity, and the more so, as the eel was not known to have any, till they were discovered by the microscope, take a piece of the skin of the eel that grows on the side, and while it is moist spread it on a piece of glass, that it may dry very smooth; when thus dried, the surface will appear all over dimpled or pitted by the scales, which lie under a sort of cuticle or thin skin; this skin may be raised with the sharp point of a penknife, together with the scales which will then easily slip out, and thus you may procure as many as you please.[43]

On the lizard, the guana, &c. are two skins; one of these is very transparent, the other is thicker and more opake; by separating these we procure two beautiful objects.

The LEAVES of many trees, and some plants, when dissected, form a very pleasing object. To dissect them, take a few of the most perfect leaves you can find, and place them in a pan with clean water; let them remain three weeks or a month without changing the water, then take them up, and try if they feel very soft, and appear almost rotten; if so, they are sufficiently soaked. You are then to lay them on a flat board, and holding them by the stalk, draw the edge of the knife over the upper side of the leaf, which will take off most of the skin; turn the leaf, and do the same with the under side. When the skin is taken off on both sides, wash out the pulpy matter, and the fibres will be exhibited in a beautiful manner. By slitting the stalk you may separate the anatomized leaf into two parts. The skins that are peeled from the fibres will also make a very good object. The autumn is the best season for the foregoing operation, as the fibres of the leaves are much stronger at that season, and less liable to break.

Ores and MINERALS should all be carefully washed and cleansed with a small brush, to remove any extraneous matter that may adhere to them. Shells may be ground down on a hone, by which their internal structure will be displayed.

To view the circulation and examine the particles of the blood. The principal part the observer must aim at, in order to view the circulation of the blood, is to procure those small animals or insects that are most transparent, that by seeing through them he may be enabled to discover the internal motion. The particular kinds best adapted for the purpose will be enumerated in the descriptive catalogue at the end of this work.

If a small eel be used for this purpose, it must be cleansed from the slime which covers it; after which it maybe put either in the fish-pan, or a glass tube filled with water, and then placed under the microscope. If the eel be small enough, the circulation may be viewed in the most satisfactory manner. Leeuwenhoeck has given, in his 112th Epistle, an accurate description of the blood vessels in part of the tail of an eel. The same figure may also be seen in my father’s Micrographia Illustrata, fourth edition, Plate XVII. The tail of any other small fish may be applied in the same manner, or tied on a slip of flat glass, and be thus laid before the microscope. Flounders, eels, and gudgeons, are to be had at almost any time in London. N. B. By filling the tube with water, when an eel is used, it will in a great measure prevent the sliminess of the eel from soiling the glass.

To view the particles of the blood, take a small drop of it when warm, and spread it as thin as possible upon a flat piece of glass. By diluting it a little with warm water, some of the larger particles will divide from the smaller, and many of them will be subdivided into still smaller; or a little drop of blood may be put into a capillary tube of glass, and be then presented before the microscope. Mr. Baker advises the mixing the blood with a little warm milk, which he says, will cause the unbroken particles to be very distinctly seen. But the most accurate observer of these particles was Mr. Hewson, and he says they have been termed globules with great impropriety, being in reality flat bodies. When we consider how many ingenious persons have been employed in examining the blood with the best microscopes, it appears surprizing that the figure of the particles should be mistaken; but the wonder is lessened when we reflect how many obvious things are overlooked, till our attention is particularly directed towards them; and besides, the blood in the human subject, and in quadrupeds, is so full of these particles, that it is with great difficulty they can be seen separate, until the blood is diluted. It was by discovering a proper method to effect this, that Mr. Hewson was indebted for his success. He diluted the particles with serum, in which they would remain undissolved, and as he could dilute them to any degree with the serum, he could easily examine the particles distinct from each other; for example, take a small quantity of the serum of the human blood, and shake a piece of crassamentum in it, till it be coloured a little with the red particles; then with a soft hair pencil spread a little of it on a piece of thin glass, and place this glass under the microscope, in such a manner as not to be quite horizontal, but rather higher at one end than the other; by which means the serum will flow from the higher to the lower extremity, and as it flows, some of the particles will be found to swim on their flat sides, and will appear to have a dark spot in the middle; others will turn over from one side to the other, as they roll down the glass.

Several authors have described an apparatus for viewing the circulation of the blood in the mesentery of a frog; but as the cruelty attendant on these kinds of investigations would deprive the humane reader of a great part of the gratification which might otherwise result from them, he will probably rest satisfied with the accounts of such experiments to be met with in authors; especially as there is an abundant variety of objects on which he may exercise his ingenuity without sacrificing the nicer feelings of humanity.[44]

OF ANIMALCULA IN INFUSIONS, &C.

These require little or no preparation. The first object is to procure them, the second, to render them visible by the microscope. A few observations, however, may be of use. Many drops of water may be examined before any can be found; so that if the observer be too hasty, he may be easily disappointed, though other parts of the same water may be fully peopled by them.

The surface of infused liquors is generally covered with a thin pellicle, which is easily broken, but acquires thickness by standing; the greatest number of animalcula are generally to be found in this superficial film.

In some cases it is necessary to dilute the infusions; but this is always to be done with distilled water, and that water should be examined in the microscope before it is made use of: the neglect of this precaution has been a source of many errors.

Animalcula are in general better observed when the water is a little evaporated, as the eye is not confused, nor the attention diverted by a few objects. To separate one or two animalcula from the rest, place a small drop of water on the glass near that of the infusion; make a small neck or gutter between the two drops with a pin, which will join them together; then the instant you perceive that an animalculum has traversed the neck or gutter, and entered the drop, cut off the communication between the two drops.

To procure the eels in paste, boil a little flower and water, till it becomes of a moderate consistence; expose it to the air in an open vessel, and beat it together from time to time, to prevent the surface thereof from growing hard or mouldy; after a few days, especially in summer time, it will turn sower, then if it be examined with attention, you will find myriads of eels on the surface.

To preserve these eels all the year, you must keep the surface of the paste moist, by putting a little water or fresh paste from time to time to the other. Mr. Baker advises a drop or two of vinegar to be put into the paste now and then. The continual motion of the eels, while the surface is moist, will prevent the paste getting mouldy. Apply them to the microscope upon a slip of flat glass, first putting on it a drop of water, taken up by the head of a pin, for them to swim in.

To make an infusion of pepper. Bruise as much common black pepper as will cover the bottom of an open jar, and lay it thereon about half an inch thick; pour as much soft water in the vessel as will rise about an inch above the pepper. The pepper and water are then to be well shaken together; after which they must not be stirred, but be left exposed to the air for a few days, when a thin pellicle will be formed on the surface of the water, containing millions of animalcula.

The observer should be careful not to form a judgment of the nature, the use, and the operations of small animalcula, from ideas which he has acquired by considering the properties of larger animals: for, by the assistance of glasses, we are introduced as it were into a new world, and become acquainted not only with a few unknown animals, but with numerous species thereof, which are so singular in their formation and habits, that without the clearest proofs even their existence would not be credited; and while they afford fresh instances of the Creator’s power, they also give further proofs of the limits and weakness of the human understanding.

DIRECTIONS FOR FINDING, FEEDING, AND PRESERVING THE POLYPES.

These little animals are to be found upon all sorts of aquatic plants, upon branches of trees, pieces of board, rotten leaves, stones, and other substances that lie in the water; they are also to be met with upon the bodies of several aquatic animals, as on the water-snail, on several species of the monoculus, &c. they generally fix themselves to these by their tail, so that it is a very good method when you are in search of the polypes, to take up a great many of these substances, and put them in a glass full of water. If there be any polypes adhering to these, you will soon perceive them stretching out their arms, especially if the glass be suffered to be at rest for a while; for the polypes, which contract themselves on being first taken out of the water, will soon extend again when they are at rest.

They are to be sought for in the corners of ditches, puddles, and ponds, being frequently driven into these with the pieces of wood or leaves to which they have attached themselves. You may, therefore, search for them in vain at one period, in a place where at another they will be found in abundance. They are more easily perceived in a ditch when the sun shines on the bottom, than at another time. In winter they are seldom to be met with; about the month of May they begin to appear and increase.

They are generally to be found in waters which move gently; for neither a rapid stream, nor stagnant waters ever abound with them. As they are always fixed to some substance by their tails, and are very rarely loose in the water, taking up water only can signify but little; a circumstance which has probably been the cause of much disappointment to those who have searched for them.

The green polypes are usually about half an inch long when stretched out; those of the second and third sort are between three quarters of an inch and an inch in length, though some are to be found at times which are an inch and a half long.

Heat and cold has the same effect upon these little creatures, that it has upon those of a larger size. They are animated and enlivened by heat, whereas cold renders them faint and languid; they should therefore be kept in such a degree of heat, that the water may not be below temperate.

It is convenient for many experiments to suspend a polype from the surface of the water. To effect this, take a hair pencil in one hand, and hold a pointed quill in the other; with the pencil loosen the polype from the receiver in which it is kept, and gradually raise it near the top of the water, so that the anterior end may be next the point of the pencil; then lift it out of the water, and keep it so for a minute; after which, thrust the point of the pencil, together with the anterior end, by little and little under water, until no more than about the twentieth part of an inch of the polype’s tail remains above its surface; at this instant, with the pointed quill remove that part of the polype from the pencil which is already in the water, at the same time blowing against the polype, by which it will be loosened, and remain out of the water.

When the polypes were first discovered, Mr. Trembley had some difficulty to find out the food which was proper for them; but he soon discovered, that a small species of the millepede answered the purpose very well: the pulices aquatices have also been recommended. The small red worms, which are to be found on the mud-banks of the Thames, particularly near the shores, answer the purpose also, they are easily found when the tide is out, when they rise in such swarms on the surface of the mud, that it appears of a red colour. These worms are an excellent food for the polype. If a sufficient quantity be gathered in November, and put into a large glass full of water, with three or four inches of earth at the bottom, you will have a supply for the polypes all the winter. They may also be fed with common worms, with the larva of gnats and other insects, and even with butcher’s meat, &c. if it be cut small enough.

River, or any soft water, agrees with them; but that which is hard and sharp prevents their thriving, and generally kills them in a few days. The worms with which they are fed should be always cleansed before you feed the polypes with them.

The polypes are commonly infested with little lice; from these it is necessary to free them, in order to preserve your polypes in a good state of health. They may be cleansed from the lice by rubbing them with a hair pencil; this cannot be easily done, unless they adhere to some substance: so that if they are suspended from the surface of the water, you must endeavour to get them to fix themselves to a piece of packthread; when they are fastened thereto, you may then rub them with a hair pencil, without loosening them from the thread.

The lice which torment the polype are not only very numerous, but they are also very large proportionably to its size: they may be said to be nearly as large with respect to them, as a common beetle is to us. If not rubbed off, they soon cover their bodies, and in a little time totally destroy them.

To preserve the polypes in health, it is also necessary often to change the water they are kept in, and particularly after they have done eating; it is not sufficient to pour the water off, all the polypes should be taken out, and the bottom and sides of the vessel rubbed from the slimy sediment adhering thereto; this is caused by their fÆces, and is fatal to them if not cleaned away. The fÆces often occasion a species of mortification, which daily increases; its progress may be stopped by cutting off the diseased part. To take them out, first loosen their tails from the sides or bottom of the glass; then take them up one by one, with a quill cut in the shape of a scoop, and place them in another glass with clean water; if they cling to the quill, let it remain a minute or two in the water, and they will soon disengage themselves.

They are preserved best in large glasses that hold three or four quarts of water; for in a glass of this size the water need not be renewed so often, particularly, if the fÆces are taken out from time to time with the feathered end of a pen, to which they readily adhere; and further, the trouble of feeding each individual is in some measure saved, as you need only throw in a parcel of worms, and let the polypes divide them for themselves.

To observe with accuracy the various habitudes, positions, &c. of this little animal, it will be necessary to place some of them in narrow cylindrical glasses; then, by means of the microscope, Fig. 3. Plate VI. you may observe them exerting all their actions of life with ease and convenience; the facility with which the lens of the fore-mentioned microscope may be moved and placed in any direction, renders it a most convenient instrument for examining any object that requires to be viewed in water.

It is also very proper to dry some of them, and place them between talcs in a slider; this, however requires some dexterity and a little practice; though, when executed with success, it fully rewards the pains of the observer. Choose a proper polype, and put it into a small concave lens, with a drop of water; when it is extended, and the tail fixed, pour off a little of the water, and then plunge it with the concave into some spirit of wine contained in the bowl of a large spoon; by this it is instantly killed, the arms and body contracting more or less; rub it gently while in the spirit with a small hair pencil, to cleanse it from the lice.

The difficulty now begins; for the parts of the polype, on being taken out of the spirit, immediately cling together, so that it is not practicable to extend the body, and separate the arms on the talc, without tearing them to pieces; therefore the only method is, to adjust them upon the talc while in the spirit: this may be done by slipping the talc under the body of the polype, while it lies in the spirit, and displaying its arms thereon by the small hair pencil and a pair of nippers; then lift the talc, with the polype upon it, out of the spirit; take hold of it with the nippers in the left hand, dip the pencil in the spirit with the right hand, and therewith dispose of the several parts, that they may lie in a convenient manner, at the same time brushing away any lice that may be seen upon the talc; now let it dry, which it does in a little time, and place the talc carefully in the hole of the slider. To prevent the upper talc and ring pressing on the polype, you must cut three pieces of cork, about the bigness of a pin’s head, and the depth of the polype, and fix them by gum in a triangular position, partly on the edges of the said talc, and partly to the sides of the ivory hole itself; the upper talc may then be laid on these corks, and pressed down by the ring as usual.[45]

[45] Baker on the Polypes.