Four years after the death of Herschel, an apothecary in the little German town of Dessau procured a small telescope, with which he began to observe the Sun. The name of this apothecary was Samuel Heinrich Schwabe (1789-1875). In 1826 he commenced to observe the spots on the Sun’s disc, counting them from day to day, more for self-amusement than from any hope of discovery; for previous astronomers had agreed that no law regulated the number of the sun-spots. Every clear day Schwabe pointed his telescope at the Sun and took his record of the spots; this he continued for forty-three years, until within a few years of his death on April 11, 1875. As early as 1843 Schwabe hinted that a possible period of ten years regulated the distribution of the spots on the Sun, but no attention was given to his idea. In 1851, however, the result of his twenty-six years of observation was published in Humboldt’s ‘Cosmos,’ and Schwabe was able to show that the spots increased and decreased in a period of about ten years. Astronomers at once recognised the importance of Schwabe’s work, and in 1857 he was rewarded by the Gold Medal of the Royal Astronomical Society of London.

Rudolf Wolf (1813-1892) of the ZÜrich Observatory now undertook to search through the records of sun-spot observation, from the days of Galileo and Scheiner, to find traces of the solar cycle discovered by Schwabe. He was successful, and was enabled to correct Schwabe’s estimate of the length of the period, fixing it as on the average 11·11 years. Additional interest, however, was given to Schwabe’s and Wolf’s investigations by the remarkable discoveries which followed. In September 1851 John Lamont (1805-1879), a Scottish astronomer,—born at Braemar in Aberdeenshire, but employed as director of the Munich Observatory,—after searching through the magnetic records collected at GÖttingen and Munich, discovered that the magnetic variations indicated a period of 10? years. Soon after this Sir Edward Sabine (1788-1883), the English physicist, from a discussion of an entirely different set of observations, independently demonstrated the same thing, proving conclusively that once in about ten years magnetic disturbances reached their height of violence; and Sabine was not slow to notice the correspondence between the magnetic period and the sun-spot period. In the same year (1852) Wolf and Alfred Gautier (1793-1881) independently made the same discovery, which had thus been made by four separate investigators.

In the same year an English amateur astronomer, Richard Christopher Carrington (1826-1875), commenced a series of solar observations which led to some remarkable discoveries. From observations on the spots, Carrington discovered that while the Sun’s rotation was performed in 25 days at the equator, it was protracted to 27½ days midway between the equator and the poles. In 1858 Carrington demonstrated the fact that spots are scarce in the vicinity of the solar equator, but are confined to two zones on either side, becoming scarce again at thirty-five degrees north or south of the equator. Contemporary with Carrington was Friedrich Wilhelm Gustav SpÖrer (1822-1895), who was born in Berlin in 1822 and died at Giessen, July 7, 1895. He commenced his solar observations about the same time as Carrington, and independently discovered the Sun’s equatorial acceleration. From observations at his little private observatory at Anclam in Pomerania, continued at the Astrophysical Observatory in Potsdam, SpÖrer demonstrated a remarkable law regarding sun-spots. This law is thus described by a well-known astronomer: “The disturbance which produces the spots of a given sun-spot period first manifests itself in two belts about thirty degrees north and south of the Sun’s equator. These belts then draw in toward the equator, and the sun-spot maximum occurs when their latitude is about sixteen degrees; while the disturbance gradually and finally dies out at a latitude of eight or ten degrees. Two or three years before this disappearance, however, two new zones of disturbance show themselves. Thus, at the sun-spot minimum there are four well-marked spot-belts,—two near the equator, due to the expiring disturbance, and two in high latitudes, due to the newly beginning outbreak.” These remarkable discoveries, which resulted from the investigations of Schwabe, Carrington, and Sporer, are a brilliant example of what may be done by amateurs in astronomy.

At the time when Carrington and SpÖrer were pursuing these researches, the spectroscope came into use as an astronomical instrument, and since 1859 solar astronomy has been almost entirely spectroscopic. Before we can rightly understand the principles of spectroscopic astronomy, we must go back to the life and work of its founder—Joseph von Fraunhofer.

The son of a poor glazier, Joseph von Fraunhofer was born on March 6, 1787, at Straubing, in Bavaria. His father and mother having died when their son was quite young, the boy, on account of his poverty, was apprenticed to a looking-glass manufacturer in Munich named Weichselberger, who acted tyrannically, keeping him all day at hard work. Still the lad borrowed some old books, and spent his nights in study. Young Fraunhofer lodged in an old tenement in Munich, which on July 21, 1801, collapsed, burying in its ruins its occupants. All were killed but Fraunhofer, who, though seriously injured, was dug out from the ruins four hours later. The distressing accident was witnessed by Prince Maximilian Joseph, Elector of Bavaria. He became interested in Fraunhofer, and presented him with a sum of money. Of this he made good use. He was already interested in optics, and he bought some books on that subject, as well as a glass-polishing machine. The remainder of the money served to procure his release from his tyrannical master, Weichselberger.

Fraunhofer became acquainted with prominent scientists at Munich, who provided him with books on optics and mathematics. Meanwhile the young optician occupied his time in shaping and finishing lenses. In 1806 he entered the optical department of the Optical and Physical Institute of Munich, and the following year, when only twenty years of age, was appointed to the chief post in that department. In 1814 he commenced his investigations with the prism, which have made his name famous.

Newton had found that, in passing through a prism, white light is dispersed into its primary colours, making up the band of coloured light known as the solar spectrum. But he failed to recognise the existence of dark lines in the spectrum. Casually seen in 1802 by William Hyde Wollaston (1786-1828), an English physicist, these lines were first thoroughly examined by Fraunhofer. Allowing light from the Sun to pass through a prism attached to the telescope, he was amazed to find several dark lines in the spectrum. By the year 1814 he had detected no less than 300 or 400 of these lines. Fraunhofer named the more prominent lines by the letters of the alphabet, from A in the red to H in the violet. They are now known as the Fraunhofer lines. At first he was much perplexed regarding the nature of the dark lines. He suspected that they might be an optical effect, depending on the quality of the glass used, and he tried different prisms, but the lines were still to be seen. Then he turned his prism to bright clouds to see if they were visible in reflected sunlight, and he found that they were. He examined the Moon and again perceived them, as moonlight is merely reflected sunlight; and they were also conspicuous in the spectra of the planets. It was thus proved that these lines were characteristic of sunlight, whether direct or reflected. It was, however, still possible that they might be caused by the passage of the rays of light from the celestial bodies through the Earth’s atmosphere. In order to test this theory, Fraunhofer examined the spectra of the brighter stars. He found that the lines visible in the solar spectrum were not to be seen in the spectra of the stars, thus proving that the lines were not merely an atmospheric effect. Each star, Fraunhofer observed, had a different spectrum from both the Sun and from other stars. These spectra were also characterised by numerous dark lines, much fainter than those in the solar spectrum.

Although he ascertained the existence of the dark lines in the Sun’s spectrum, Fraunhofer never really found out what they represented. As Miss Giberne expresses it, “Although he now saw the lines he could not understand them: he could not read what they said. They spoke to him indeed about the Sun, but they spoke to him in a foreign language, the key to which he did not possess.” However, he expressed the belief that the pair of lines in the solar spectrum, which he marked D, coincided with the pair of bright lines emitted by incandescent sodium. Although he doubtless suspected that the lines conveyed intelligence regarding the elements in the Sun, he never was able properly to decipher their meaning. Had he lived, he would probably have made the great discovery; but these investigations were cut short by his sudden and untimely death on June 7, 1826.

After the death of Fraunhofer, very little was done to forward the study of spectrum analysis. Investigations in this branch of research were made, however, by Sir John Herschel (1792-1871), William Allen Miller (1817-1870), Sir David Brewster (1781-1868), and others. Two famous men of science had partly discovered the secret. These were Sir George Stokes (1819-1903), of Cambridge, and Anders John AngstrÖm (1812-1872) of Upsala. Of AngstrÖm’s work, published in 1853, it has been said that it would “have obtained a high celebrity if it had appeared in French, English, or German, instead of Swedish.”

It was not until 1859 that the principles of spectrum analysis were fully enunciated by Gustav Robert Kirchhoff (1824-1887), and his colleague in the University of Heidelberg, Robert Wilhelm Bunsen (1811-1899). Kirchhoff demonstrated that a luminous solid or liquid gives a continuous spectrum, and a gaseous substance a spectrum of bright lines. In the words of Miss Clerke, “Substances of every kind are opaque to the precise rays which they emit at the same temperature. That is to say, they stop the kinds of light or heat which they are then actually in a condition to radiate.... This principle is fundamental to solar chemistry. It gives the key to the hieroglyphics of the Fraunhofer lines. The identical characters which are written bright in terrestrial spectra are written dark in the unrolled sheaf of sun-rays.” Kirchhoff made several determinations of the substances in the Sun, proving the existence of sodium, iron, calcium, magnesium, nickel, barium, copper, and zinc. His great map of the solar spectrum was published by the Berlin Academy in 1860, and represented an enormous amount of labour. It was succeeded by another map by AngstrÖm, published in 1868. But both of these maps have been recently superseded by the investigations of Sir Joseph Norman Lockyer (born 1836), and of the American physicist, Henry Augustus Rowland (1848-1901). Rowland largely increased our knowledge of the elements in the solar atmosphere.

The spectroscope had become, by 1868, a recognised instrument of astronomical research, and in that year it was applied during the famous total eclipse, visible in India. There were many eclipse problems, arising from the observations made by the eclipse expeditions of 1842, 1851, and 1860. The eclipse of 1851 had finally proved that the red flames seen surrounding the Sun during total eclipses belonged to the Sun, and not to the Moon, as many astronomers had believed. At the eclipse of 1860, visible in Spain, the Italian astronomer, Angelo Secchi (1818-1878), and the Englishman, Warren De la Rue (1815-1889), secured photographs of the solar prominences. The problem of 1868 was the constitution of these prominences.

Pierre Jules CÉsar Janssen, born in Paris in 1824, was stationed at Guntoor, in India, to observe the eclipse. He succeeded in observing the spectrum of the prominences during the progress of totality, and found it to be one of bright lines, proving the gaseous nature of the sun-flames. During the progress of the eclipse, Janssen was specially struck by the brilliancy of the bright lines, and it occurred to him that the prominence-spectrum could be observed in full daylight, if sufficient dispersive power was used to enfeeble the ordinary continuous spectrum. At ten o’clock on the following morning, August 19, 1868, Janssen applied his spectroscope to the sun, and observed the prominence-spectrum. After a month’s observation in India, he sent to the French Academy an account of his success. A short time, however, before his report arrived, the Academy had received a similar one from Lockyer, who had independently made the same discovery. Two years previously, in 1866, the new method had occurred to him, but his spectroscope was not powerful enough; and although he ordered a more powerful one at once, it was not until October 16, 1868, that he had the instrument in his hands. Four days later he observed the prominence-spectrum in full daylight.

The next advance in the study of the prominences was announced in 1869. Janssen and Lockyer had shown astronomers how to observe the spectrum of the prominences; but the researches of other two famous astronomers enabled observers to see the forms of the prominences. These two men were William Huggins (born 1824) and Johann Carl Friedrich ZÖllner. The latter astronomer, born in Leipzig in 1834, was one of the most successful students of the solar prominences. He was Professor of Astrophysics in the University of Leipzig, a position which he filled with success until his untimely death on April 25, 1882. Independently of Huggins, he found that by opening the slit of the spectroscope wider, the forms of the prominences themselves could be seen. The study of the prominences was at once taken up by the most famous solar observers: these were Huggins and Lockyer in England, SpÖrer and ZÖllner in Germany, Janssen in France, Secchi, Respighi, and Tacchini in Italy, Young in America. To Charles Augustus Young (born 1834) we owe the careful study of individual prominences. On September 7, 1871, he observed the most gigantic outburst on the sun ever witnessed, fragments of an exploded prominence reaching a height of 100,000 miles: Young, also, made the first attempt to photograph the prominences.

To the Italian school of astronomers, however, we owe the persistent and systematic study of the prominences. Among them the three greatest names are Angelo Secchi (1818-1878), Lorenzo Respighi (1824-1889), and Pietro Tacchini (1838-1905). After the death of Secchi, the recognised head of spectroscopy in Italy was Pietro Tacchini. Born at Modena in 1838, he was appointed director at Modena in 1859, assistant at Palermo in 1863, and director at Rome in 1879. In 1870 he commenced to take daily observations of the prominences, noting their sizes, forms, and distribution, and these observations were continued for thirty-one years, until within four years of Tacchini’s death, which took place on March 24, 1905. Tacchini did for the study of the prominences what Schwabe did for the spots. The Italian spectroscopists found that the prominences increased and decreased every eleven years in harmony with the spots. Tacchini demonstrated that the streamers of the solar corona originate in regions where the prominences are most numerous, and that the shape of the corona, on the whole, varies in sympathy with the prominences.

The researches of Lockyer indicated that the prominences originated in a shallow gaseous atmosphere which he termed the chromosphere. Formerly astronomers had to observe only isolated prominences, but in 1892 an American astronomer, George Ellery Hale (born 1868), formerly director of the Yerkes Observatory, and now director of the Solar Observatory in California, succeeded in photographing, by an ingenious process, the whole of the chromosphere, prominences, and faculÆ visible on the solar surface.

Another solar envelope was discovered in 1870 by Dr Charles Augustus Young, who from 1866 to 1877 directed the Observatory at Dartmouth, New Hampshire, and from 1877 to 1905, that at Princeton, New Jersey. During the eclipse of December 22, 1870, Young was stationed at Tenez de Frontena, Spain. As the solar crescent grew apparently thinner before the disc of the Moon, “the dark lines of the spectrum,” he says, “and the spectrum itself gradually faded away, until all at once, as suddenly as a bursting rocket shoots out its stars, the whole field of view was filled with bright lines, more numerous than one could count. The phenomenon was so sudden, so unexpected, and so wonderfully beautiful, as to force an involuntary exclamation.” The phenomenon was observed for two seconds, and the impression was left on the astronomer that a bright line had taken the place of every dark one in the solar spectrum, the spectrum being completely reversed. Hence the name which was given to the hypothetical envelope—“the reversing layer.” For long the existence of the reversing layer was disputed by numerous astronomers. In 1896 photographs taken during the solar eclipse of that year finally demonstrated the existence of the “flash spectrum” as seen by Young.

The last of the solar appendages, the corona, can only be seen during total eclipses. The researches of Young and Janssen indicate that it is partly gaseous and partly meteoric in its constitution; and various photographs, taken at the eclipses since 1870, have demonstrated its variation in shape, which is in harmony with the eleven-year period. Several attempts have been made to observe the corona without an eclipse. In 1882 Huggins made the attempt, but failed, and Hale, with his photographic process, had no better success. More recently, in 1904, a Russian astronomer, Alexis Hansky, observing from the top of Mont Blanc, secured some photographs on which he believes the corona is represented, but so far his observations have not been confirmed by other astronomers.

The application of the spectroscope to the motions on the solar surface is perhaps one of the most wonderful triumphs in astronomical science. In 1842 Christian Doppler (1803-1853), Professor of Mathematics at Prague, had expressed the view that the colour of a luminous body must be changed by its motion of approach or recession. It was obvious to Doppler that if the body was approaching, a larger number of light waves must be entering the eye of the observer than if it were retreating. Miss Clerke thus illustrates Doppler’s principle: “Suppose shots to be fired at a target at fixed intervals of time. If the marksman advances, say, twenty paces between each discharge of his rifle, it is evident that the shots will fall faster on the target than if he stood still; if, on the contrary, he retires by the same amount, they will strike at correspondingly longer intervals.” It occurred to various astronomers that it would be possible to measure cyclones and hurricanes in the Sun, not by change of colour in the spectrum, but by the shifting of the lines; and in 1870 this was successfully done by Lockyer. In the next few years efforts to measure the solar rotation were made by Young, ZÖllner, and others, who succeeded in measuring the displacement of the lines, but not the time of rotation. This was reserved for the famous Swedish astronomer, DunÉr.

Nils Christopher DunÉr, born in 1839 in Scania, was employed as an assistant at Lund Observatory from 1858 to 1888, when he was appointed director of the Observatory at Upsala. In that year he commenced a study of the solar rotation, measuring it by means of Doppler’s principle. He confirmed the telescopic work of Carrington and SpÖrer on the equatorial acceleration, and measured the displacement up to within fifteen degrees of the poles. He brought out the surprising fact that the rotation period of the Sun is there protracted to 38½ days. These remarkable researches were published in 1891.

In 1873 the Astronomer-Royal of England commenced at Greenwich Observatory to photograph the Sun daily. This work has been carried on there by Edward Walter Maunder (born 1851), and Greenwich Observatory possesses a photographic record of sun-spots. At the Meudon Astrophysical Observatory, near Paris, Janssen has, since 1876, secured photographs of the solar surface, which were comprised in a great atlas, published by him in January 1904. These photographs have revealed a remarkable phenomenon—the “rÉseau photospherique,” the distribution over the solar surface of blurred patches of light, which Janssen considers are inherent in the Sun. The Greenwich records of sun-spots and of magnetic disturbances have been made use of by Maunder in his remarkable studies, promulgated in 1904, of the connection between sun-spots and terrestrial magnetism. Maunder finds that on the average magnetic storms are dependent on the presence of sun-spots, and on the size of the spot. The magnetic action, he finds, does not radiate equally in all directions from the sun-spots, but along definite and restricted lines.

Herschel’s hypothesis of a dark and cool globe beneath the solar photosphere was seen to be untenable after the introduction of the spectroscope. The first important theory as to the solar constitution was that advanced in 1865 by the French astronomer, HervÉ Faye (1814-1902). Numerous other theories were afterwards advanced by Secchi, ZÖllner, Young, and others, but a complete description of the various developments in solar theorising cannot be given here. There is no complete “theory” of the exact constitution of every part of the Sun, but the unpretentious “Views of Professor Young on the Constitution of the Sun,” which appeared in April 1904 in ‘Popular Astronomy,’ represent the latest ideas of the foremost solar investigator. Professor Young regards the reversing layer and the chromosphere as “simply the uncondensed vapours and gases which form the atmosphere in which the clouds of the photosphere are suspended.” He says that the contraction theory of Helmholtz,—explained in another chapter,—advanced to explain the maintenance of the Sun’s heat, is true so far as it goes; but that it is all the truth is now made doubtful by the discovery of radium, which “suggests that other powerful sources of energy may co-operate with the mechanical in maintaining the Sun’s heat.”

The important question of the distance of the Sun was thoroughly investigated in 1824 by Johann Franz Encke (1791-1865), then of Seeberg, near Gotha, who, from a discussion of the transits of Venus in 1761 and 1769, found a parallax of 8·571, corresponding to a mean distance of 95,000,000 miles. This value was accepted for thirty years, until Peter Andreas Hansen (1795-1874), in 1854, and Urban Jean Joseph Le Verrier (1811-1877), in 1858, found from mathematical investigations that the distance indicated was too great. Preparations were accordingly made for the observation of the transits of Venus, which took place respectively on December 8, 1874, and December 6, 1882. On the first occasion many expeditions were sent to view the transit, consisting of French, German, American, English, Scottish, Italian, Russian, and Dutch astronomers, and it was hoped that the solar parallax would be accurately measured once for all. However, the transit, although favoured with good weather, was not successful, owing to the difficulty of making exact measurements, by reason of the illumination and refraction in the atmosphere of Venus. Accordingly the values deduced for the parallax were far from unanimous. The transit of 1882 was not observed so extensively, as astronomers had found the transit of Venus to be by no means the best method. In 1877 Sir David Gill (born 1843), the great Scottish astronomer, determined the solar parallax successfully from measures of the parallax of Mars in opposition. His value was 8·78, corresponding to 93,080,000 miles. Some years previous to this Johann Gottfried Galle (born 1812), the German astronomer, had, from measurements of the parallax of the asteroid Flora, deduced a solar parallax of 8·87. Gill’s work at the Cape in 1888, on the Asteroids, was successful in giving a more accurate value than the transit of Venus: in 1900 and 1901 measures of the parallax of the asteroid Eros, the nearest minor planet, were made by many different observatories, and agree with the other results. The values which have been derived from the velocity of light, and from the constant of aberration, are fairly in agreement with those derived from direct measurement. On the whole, the most probable value of the parallax is about 8·8, indicating a mean distance of about 92,700,000 miles, with a “probable error” of about 150,000 miles.

What a different picture the sun presents to us at the beginning of the twentieth century from that which it presented to Herschel and his contemporaries at the beginning of the nineteenth! To Herschel, the Sun was a cool dark globe, surrounded by a luminous atmosphere. As the outcome of the researches and discoveries outlined in this chapter, the Sun is now seen to be a vast central world, which is over a million times larger than the Earth. In the words of an able writer, “It is most probably a world of gases, where most of the metals and metallic gases that we know exist only as vapours, even at the Sun’s surface, hotter than any furnace on earth, and getting a still fiercer heat for every mile of descent lower. Of that heat in the Sun’s interior we can form no conception. The pressure within the Sun is equally inconceivable. A cannon-ball weighing 100 lb. on earth would weigh 2700 on the Sun. Thus a mighty conflict goes on unceasingly between imprisoned and expanding gases and vapours struggling to burst out, and massive pressures holding them down. For reasons we cannot fully understand, no equilibrium is reached. For millions of years up-rushes and down-rushes of the white-hot materials have been proceeding on that bright photosphere which gives us light, and looks a picture of calm and quiescence. Above that is a comparatively thin rose-coloured layer, the chromosphere, agitated with fiery ‘prominences,’ and outside all these the coronal glory—all alike pointing to immeasurable activities.”

The following remark of Professor Newcomb shows our inability to realise the solar activity. “Suppose,” he says, “every foot of space in a whole country covered with 13-inch cannon, all pointed upward, and all discharged at once. The result would compare with what is going on inside the photosphere about as much as a boy’s popgun compares with the cannon.”