William Herschel and his work/Chapter 9
So carefully and persistently was the sun studied by Herschel that, for the sake of clearness, it is advisable to arrange his work not in the order of time, but according to the subject he treats of. He began at an early period to watch the sun's face, and to make experiments with the view of discovering its history, past and future. Could he but read that history or even a chapter of it, he felt that he would be able to read the history of other suns as well as ours, and perhaps to lay a foundation for fellow-labourers in the same cause to build a temple to science on. He succeeded beyond his wishes, or at least his hopes.
The first thing he endeavoured to ascertain was, whether the sun was stationary or nearly stationary in the heavens. Astronomers had already discovered that its immense fiery globe had a day like our earth, that is, that it turned round on its axis precisely as the earth does. The time it takes they found to be 25d 7h 48m of our reckoning. This is the length of the sun's day. But Herschel asked if the sun had not a year as well as a day, a time—vast, immeasurable, perhaps—in which it revolves round a centre, hidden from man's knowledge, but not from man's sight, if he only knew where to look for it. Herschel looked for an unknown centre. He did not find it, but he believed, as we have already seen, first, that the sun was moving among the stars, and second, that it was moving towards a spot in the constellation Hercules in the northern sky.
As the sun is the source of light and heat, and both of them had to be considered in his observations, it was natural that Herschel should turn his thoughts to the solar spectrum, as we call what is commonly spoken of as the rainbow. A glass prism produces the same effect on a beam of sunlight as a raindrop or a cloud curtain composed of millions of them: it divides or decomposes the white light of one sun into that of seven suns of different colours, red, orange, yellow, green, blue, indigo, violet, and it also bends or refracts them from the straight line the sunbeam would otherwise pursue. The red is the least bent, the violet most. By the refraction or bending is meant what is seen by thrusting one half of a walking-stick into water, and keeping the other half out of it in the air. But it happened that in shielding his eye from the sun when looking at its disc through a telescope, Herschel had used glass of various colours to dim the glare and heat. This experience was fatal to the use of glass coloured red. "I began with a red glass," he says, "and, not finding it to stop light enough, took two of them together. These intercepted full as much light as was necessary; but I soon found that the eye could not bear the irritation, from a sensation of heat, which, it appeared, these glasses did not stop. I now took two green glasses: but found that they did not intercept light enough. I therefore smoked one of them: and it appeared that, notwithstanding they still transmitted considerably more light than the red glasses, they remedied the former inconvenience of an irritation arising from heat. Repeating these trials several times, I constantly found the same result." How to see the sun distinctly without inconvenience or danger from the heat continued to occupy his thoughts for years. "I viewed the sun through water," he wrote in 1801. "It keeps the heat off so well, that we may look for any length of time, without the least inconvenience." "Ink diluted with water gave an image of the sun as white as snow; and I saw objects very distinctly, without darkening glasses."
Herschel introduced his papers on the sun's light and heat with a wise remark, which proved him to be as good an observer in the world of mind as in that of matter. "It is sometimes of great use in natural philosophy to doubt of things that are commonly taken for granted; especially as the means of resolving any doubt, when once it is entertained, are often within our reach. . . . It will therefore not be amiss to notice what gave rise to a surmise, that the power of heating and illuminating objects might not be equally distributed among the variously coloured rays." The experiments, which he then made on the light and heat given out by each colour of the spectrum, were admirably imagined and beautifully carried out. He was really engaged on a continuation of Newton's experiments on sunbeams, but the field of research was new and untrod. Gradually the questions to which he sought answers began to take shape more distinctly in his mind. When a prism intercepts a beam of sunlight, let into a darkened room through a hole in the window shutter, and the band of coloured light, five times as long as it is broad, falls on a screen placed behind the prism, is the whole band equally heated or equally luminous? and is the whole sunbeam found decomposed into the colours seen? Regarding equality of heating and illumination in the various colours, Herschel's experiments made it plain that at the red end there are visible rays, which are hotter than those in any other part of the coloured band or spectrum. The heat he found diminishing as the refrangibility increases from the red to the violet end. The power of illuminating an object, on the contrary, increases from the red to the orange, from the orange to the yellow, and reaches its greatest intensity between the yellow and the green, after which it rapidly decreases in the blue, more so in the indigo, till it becomes "very deficient in the violet." One of his experiments, by the help of a microscope, was with a guinea:—"Red showed four remarkable points: very distinct. Orange, better illuminated: very distinct. Yellow, still better illuminated: very distinct: the points all over the field of view are coloured; some green; some red; some yellow; and some white, encircled with black about them. Between yellow and green is the maximum of illumination: extremely distinct. Green, as well illuminated as the yellow: very distinct. Blue, much inferior in illumination: very distinct. Indigo, badly illuminated: distinct. Violet, very badly illuminated: I can hardly see the object at all."
His second inquiry was. Is a sunbeam passing through a prism and received on a screen behind it represented entirely by the coloured and visible band of the spectrum? His answer to this question was a distinct no, and a hinted suspicion that the no extended or might extend farther than it was in his power to prove. He could and did show that a thermometer rose in passing from the violet to the red end of the spectrum: but he did more. He placed the thermometer beyond the visible red, and found that, as it continued to rise, heat-rays, invisible to the eye and less bent from the straight path of the sunbeam, gave the greatest heat. He must have asked himself. Is there not something similar at the violet end; but he had not the means of answering the question. He did what was next best. He asked a question pregnant with great results, and destined to bear an abundant harvest for the welfare and instruction of man. "It may be pardonable if I digress for a moment, and remark, that the foregoing researches ought to lead us on to others. May not the chemical properties of the prismatic colours be as different as those which relate to light and heat: . . . they may reside only in one of the colours." To this question he could neither give nor get an answer. A short time passed, and the answer came from Germany and, independently, from England. "The existence of solar rays accompanying light, more refrangible than the violet rays, and cognisable by their chemical effects, was first ascertained by Mr. Ritter." They were called "The dark rays of Ritter," and "appeared to extend beyond the violet rays of the prismatic spectrum, through a space nearly equal to that which is occupied by the violet." "Paper dipped in a solution of nitrate of silver" was used to prove the existence of these chemical rays and to introduce the days of photography. It was most fitting that it should be so. An astronomer led the way in this new quest after invisible rays; chemistry supplemented his discoveries by paving the way for photography, and paid back its debt to astronomy by shortening the processes of its art, and faithfully recording the face of the heavens, as the most skilful draughtsman could not do. Truly, Herschel was a seer, whose imagination captured truth, though men less gifted mocked him as a dreamer. The equerry in Windsor Castle was justified in assuring Miss Burney that time would do justice to Herschel, as it had done to Newton.
Herschel's mistakes, in his subsequent inquiries, arose largely from his belief in Newton's theory that light-giving bodies, like the sun, emit infinitely small particles, which enter the eye and affect the retina so as to produce vision. Hence he spoke of the momenta of these particles. His contemporary, Dr. Thomas Young, maintained that light, like air, was produced by waves propagated at a vast rate of speed, and in immensely short lengths, through a universally diffused and infinitely rare medium, called ether A Frenchman, Fresnel, has got most of the credit of establishing this theory. But the third question asked and answered by Herschel in these papers about the sun was. Is light the same or different from heat? His experiments were carefully arranged and as carefully made, and the conclusion reached was that they are different. He also wrote two long papers on the coloured rings produced when two watch-glasses, or one and a plane glass, are pressed together so as to leave a thin plate of air between them. Amid undoubtedly excellent observations he was too hasty in what he then wrote, and too rash in the conclusions he then drew. But let it be recorded to his honour that to him belongs the credit of first sending the beams of Sirius and other sunny stars through a prism, for the purpose of determining whether their light is like our sun's or not. It was a most brilliant idea, carried out before the world was ready to receive it.
The great question Herschel set himself to solve regarding the sun was, What is it? He knew, as all men had known, that it was a vast fiery ball ruling earth and sky; but he saw, as they saw, nothing save the outside of the ball. Was it a mighty furnace within as it was without? In Newton's days, two or three generations earlier, there were people who "supposed the sun to be cold," although Newton easily showed that, to "a body hard by the sun, his heat would be 50,000 times greater than we feel it in a hot smnmer day, which is vastly greater than any heat we know on earth." Herschel was aware that the spots, the black spots on its face, were vast dark holes in its white brightness, so large that they would let the earth dive in, and be at a thousand miles' distance all round from the burning, blazing clouds. But while he knew this, he had also learned from the writings of others that these black rifts were careering over its face from west to east at the rate of more than a mile every second. What did it all mean, was the question he wished answered. Fabricius in 1611, and Galileo about the same time, divide between them the honour of discovering these spots on the sun's face. The former tells the story of his first sight of a spot, of his own and his father's keenness in viewing it till the heat affected their eyes, of his extreme impatience till morning again revealed to him in the sun itself what he thought was only a cloud, and of the incredible delight with which he welcomed the strange stain on the sun's brightness, but removed a little from the place where it was seen the day before—he tells a true story with the pen of a romancer inventing a world of wonders. The darkened room, the hole in the shutter, the sheet of white paper to receive the bright image, and the sun's rotation on his axis then burst upon the world in his pages.
Some imagined that these vast fields of darkness were smoke from gigantic volcanoes on the sun; others considered them to be a mighty expanse of scum floating on a burning ocean, or dark clouds swimming in highly heated gas. But Herschel's telescope told him they were immense pits dug somehow in the shining and fiery brightness, while waves of fiercer brightness surged round the edges in crests of vast height, for which the name faculæ, or torches, had been long before invented. Over many million of square miles of the sun's surface this rising of fiercely heated waves and this digging out of black hollows were continually going on in a greater or lesser degree. As many as forty of the latter were once seen by Herschel, when he was watching Mercury, so to speak, picking his way amongst them during his passage across the sun's disc. Other observers laid claim to counting no fewer than fifty at one and the same time. What were they? In July 1643 Hevelius saw a procession of spots and bright crests more than a third of the sun's surface in length, or nearly twice as far as the distance of the moon from the earth! Then spots were seen of such a depth that when they reached the sun's edge they made a notch on the rim. It was evident they were not volcanoes spouting forth solid matter to immense heights and blackening with solar smoke the photosphere, as Schroeter called the envelope of light which clothed the sun. They were not dark bodies like planets circling round this fiery ball. Nor were they masses of black scum floating on an ocean of brightness. In 1779 Herschel saw a great spot which appeared to be divided into two parts. One of them was more than thirty-one thousand miles in length, the other was about twenty thousand, and a ridge of shining light separated the one from the other. Four years later he observed another, "a fine large spot," and followed it to the edge of the sun. He came to the conclusion that he was looking into a vast pit, with "very broad, shelving sides," on to "the real solid body of the sun itself." Eight years after, in 1791, he came to the same conclusion regarding another large spot: it was a pit below the level of the bright surface; round the dark part it had a broad margin less bright than the surface, and also lower down. Accompanying the spots were the faculæ, as Hevelius called "the ridges of elevation above the rough surface" of the sun. "About all the spots the shining matter seemed to have been disturbed; and was uneven, lumpy, and zigzagged in an irregular manner." These waves or ridges of brightness are of immense extent, but Herschel objected to call them torches, as "they appeared like the shrivelled elevations on a dried apple, extended in length, and most of them joined together, making waves, or waving lines." In 1801 he had advanced to the "strong suspicion that one half of our sun is less favourable to a copious emission of rays than the other; and that its variable lustre may possibly appear to other solar systems, as irregular periodical stars are seen by us" In the same paper he records in his observations that he counted at one time 45 "openings" or spots, on the following day 50, and three days later above 60. A cloud, hanging over one of these openings, was seen to move a third of the way across the mighty chasm in fifty-eight minutes.
Herschel's theory of the sun then may be thus stated. There is first the region of "luminous solar clouds" which, adding also the elevation of the faculæ, cannot be less than 1843, nor much more than 2765 miles in depth. These solar clouds he compares in density with the aurora borealis of our skies. Underneath this envelope of brightness is the sun's atmosphere, which may be so clouded as to shield the body of the sun and the beings, who live there, from the intense heat and glare above. The body of the sun lies still lower, and "is diversified with mountains and valleys." Some may deem it the horrid abode of lost souls; others may see in its cool retreats the home of blessed spirits. But so imbued is man's mind with the idea of unbearable heat in the sun that, in a court of law, belief in its coolness was at that time quoted as a proof of insanity, and of incompetence in a man to manage his own affairs. This, in short compass, is Herschel's view of the constitution of the sun. It is largely founded on the theory of his friend Wilson, the Professor of Astronomy in the University of Glasgow. So far as spots are concerned, it works out to an attractive and popular resemblance to truth. Suppose a disturbance—call it hurricane or tornado—to take place in the solar atmosphere. Everything is on a gigantic scale, mountains, winds, waves in this ocean of light A mighty updraft from below rolls back, for a longer or shorter time, the luminous solar clouds. Into the vast pit thus laid open these clouds pour a flood of light on the body and cloudy atmosphere of the sun. The former looks black against the light, but reveals mountains upwards of three hundred miles in height; the latter, with its shelving sides, returns more of the light, and is less black; while the shining matter, rolled back into waves of enormous length and height, is heaped up in fiery storms round the vast gulf. The dark body of the sun is called the macula, or spot; the better lighted atmospheric shield, the penumbra; and the heaped-up waves the faculæ, which give the sun's surface the roughness of aspect it presents.
This was all that Herschel saw or imagined. It was far within the truth for awe-inspiring beauty, and for the gigantic movements of these "luminous solar clouds." Had he seen the "blood-red streak" of the total eclipse of 1706, or the "corona" and "the ruddy clouds" of that of 1715, the science of astronomy would have been perhaps half a century in advance of the position he left it in at his death. He did not see either blood-red streak or corona. There is no reason to believe that he even read of them. His pigmy mountains of three or four hundred miles were molehills to the vast tongues of red flame shot up from the burning ocean of the sun's surface to a height of 200,000 miles in a few minutes, rising from and falling back into that ocean's bosom in a couple of hours. Herschel would have revelled in these gigantic strides of living flame. He would have cast away his theory of solid body, atmosphere and luminous solar clouds. Probably he would have held fast to his comparison of the light-clouds to our northern lights, and to his idea that the comets help to maintain the light and heat of our sun. How his glory is kept up from age to age, from millennium to millennium, we know as little as he did. Truly we are only at the beginning of our knowledge of this and other glorious stars; Herschel may have thought, and probably did think, that we were nearly at the end.
- Dr. Young, "The Bakerian Lecture, Phil. Trans., for 1802, pp. 14, 15, "A luniniferous ether pervades the universe, rare and elastic in a high degree." He was well abused by an Edinburgh Reviewer for this Lecture.
- Brewster, Life, ii. 455.
- Scots Magazine, 1807, p. 329.
- Had Herschel known and reflected on the letter of Sir Isaac Newton printed in his Life, ii. 455, he would probably not have published this theory. "The whole body of the sun, therefore, must be red-hot" is Newton's conclusion. Even then it would look black against the surface luminous clouds.