This hypothesis is considered by the most competent judges to render a very satisfactory account of the facts. But it has not been universally adopted. Some writers of authority have lately represented the spots as scoriæ floating on a liquid surface, and ejected from solar volcanoes, of which the burning mountains of the earth convey but a feeble idea. Hence observations become necessary as to the nature of the incandescent matter of the sun; and when we remember the immense distance of that body, such an attempt may well appear to be one of temerity.
The progress of optical science, however, has given us the means of determining this apparently insoluble question. It is well known, that physicists are enabled at present to distinguish two kinds of light—natural light and polarised light. A ray of the former exhibits the same properties on any part of its form; not so the latter. A polarised ray is said to have sides, and the different sides have different properties, as demonstrated by many interesting phenomena. Strange as it may seem, these rays thus described as having sides, could pass through the eye of a needle by hundreds of thousands without disturbing each other. Availing themselves, therefore, of the assistance of polarised light, and an instrument named the polariscope, or polarising telescope, observers obtain a double image of the sun, both alike, and both white; but on reflecting this image on water, or a glass mirror, the rays become polarised; the two images are no longer alike or white, but are intensely coloured, while their form remains unchanged. If one is red, the other is green, or yellow and violet, always producing what are called the complementary colours. With this instrument, it becomes possible to tell the difference between natural and polarised light.
Another point for consideration is, that for a long time it was supposed, that the light emanating from any incandescent body always came to the eye as natural light, if in its passage it had not been reflected or refracted. But experiment by the polariscope shewed, that the ray departing from the surface at an angle sufficiently small was polarised; while at the same time, it was demonstrated that the light emitted by any gaseous body in flame—that of street-lamps, for instance—is always in the natural state, whatever be its angle of emission. From these remarks, some idea will be formed of the process necessary to prove whether the substance which renders the sun visible is solid, liquid, or gaseous. On looking at the sun in the polariscope, the image, as before observed, is seen to be purely white—a proof that the medium through which the luminous substance is made visible to us is gaseous. If it were liquid, the light would be coloured; and as regards solidity, that is out of the question—the rapid change of spots proves that the outer envelope of the sun is not solid. On whatever day of the year we examine, the light is always white. Thus, these experiments remove the theory out of the region of simple hypothesis, and give certainty to our conclusions respecting the photosphere.
Here an example occurs of the aids and confirmations which science may derive from apparently trivial circumstances. Complaint was made at a large warehouse in Paris, that the gas-fitters had thrown the light on the goods from the narrow, and not from the broad side of the flame. Experiments were instituted, which proved that the amount of light was the same whether emitted from the broad or narrow surface. It was shewn also, that a gaseous substance in flame appears more luminous when seen obliquely than perpendicular, which explains what are known as faculæ and lucules, being those parts of the solar disk that shew themselves brighter than other portions of the surface. These are due to the presence of clouds in the solar atmosphere; the inclined portions of the clouds appearing brightest to the spectator. The notion, that there were thousands on thousands of points distinguishing themselves from the rest by a greater accumulation of luminous matter, is thus disposed of.
Still, there remained something more to be determined. The existence of the photosphere being proved, the question arose—was there nothing beyond? or did it end abruptly? and this could only be determined at the period of a total eclipse, at the very moment when the obscuration of the sun being greatest, our atmosphere ceases to be illuminated. Hence the interest felt in an eclipse of the sun of late years.
In July 1842, at a total eclipse of the sun visible in several parts of the continent, the astronomers noticed, just as the sun was hidden by the moon, certain objects, in the form of rose-coloured protuberances, about two or three minutes high, astronomically speaking, projected from the surface of the moon. These appearances were variously explained: some supposed them to be lunar mountains; others saw in them effects of refraction or diffraction; but no precise explanation could be given; and mere guesses cannot be accepted as science. Others, again, thought them to be mountains in the sun, the summits stretching beyond the photosphere; but at the most moderate calculation, their height would have been about 60,000 miles—an elevation which, as is said, the solar attraction would render impossible. Another hypothesis was, that they were clouds floating in a solar, gaseous atmosphere.
M. Arago considers the last as the true explanation: it remained the great point to be proved. If it could be ascertained, that these red protuberances were not in actual contact with the moon, the demonstration would be complete. Speculation was busy, but nothing could be done in the way of verification until another eclipse took place. There was one in August 1850 total to the Sandwich Islands, at which, under direction of the French commandant at Tahiti, observations were made, the result being that the red prominences were seen to be separated by a fine line from the moon's circumference. Here was an important datum. It was confirmed by the observations of July 1851, by observers of different nations at different localities, who saw that the coloured peaks were detached from the moon; thus proving that they are not lunar mountains.
If it be further ascertained, that these luminous phenomena are not produced by the inflexion of rays passing over the asperities of the moon's disk, and that they have a real existence, then there will be a new atmosphere to add to those which already surround the sun; for clouds cannot support themselves in empty space.
We come next to that part of the subject which treats of the true place of the sun in the universe. In the year 448 b.c., Archelaüs, the last of the Ionian philosophers, without having made any measurements, taught that the sun was a star, but only somewhat larger than the others. Now, the nearest fixed star is 206,000 times further from us than the sun: 206,000 times 95,000,000 of miles—a sum beyond all our habits of thought. The light from the star Alpha of the Centaur is three years in its passage to the earth, travelling at the rate of 192,000 miles per second; and there are 86,400 seconds in a day, and 365 days in a year. Astounding facts! If the sun, therefore, were removed to the distance of a Centauri, its broad disk, which takes a considerable time in its majestic rising and setting above and below the horizon, would have no sensible dimensions, even in the most powerful telescopes; and its light would not exceed that of stars of the third magnitude—facts which throw the guess of Archelaüs into discredit. If our place in the material universe is thus made to appear very subordinate, we may remember, as M. Arago observes, that man owes the knowledge of it entirely to his own resources, and thereby has raised himself to the most eminent rank in the world of ideas. Indeed, astronomical investigations might not improperly excuse a little vanity on our part.'
Among the stars, Sirius