Sir William Herschel: His Life and Works Part 9

The mirror and the rest of the polishing apparatus are on the premises. The iron grinding tools and polishers are placed underneath the tube, let into the ground, and level with the surface of the gravelled area in which it stands.". . .

The closing of the tube was done with appropriate ceremony on New-Year's-Day, 1840, when, after a procession through it by the family at Slough, a poem, written by Sir JOHN, was read, the machinery put into its present position, and the tube sealed.

The memoir on the forty-foot telescope shows throughout that HERSCHEL'S prime object was not the making of the telescope itself, but that his mind was constantly directed towards the uses to which it was to be put--towards the questions which he wished it to answer.

Again and again, in his various papers, he returns to the question of the _limit of vision_. As BESSEL has said:

"The naked eye has its limit of vision in the stars of the sixth magnitude. The light of fainter stars than these does not affect the retina enough for them to be seen. A very small telescope penetrates to smaller, and, in general, without doubt, to more distant stars.

A more powerful one penetrates deeper into s.p.a.ce, and as its power is increased, so the boundaries of the visible universe are widened, and the number of stars increased to millions and millions. Whoever has followed the history of the series of HERSCHEL'S telescopes will have observed this. But HERSCHEL was not content with the bare fact, but strove ever to know _how far_ a telescope of a certain construction and size could penetrate, compared with the naked and una.s.sisted eye. These investigations were never for the discovery of new facts concerning the working of his instruments; it was for the knowledge of the distribution of the fixed stars in s.p.a.ce itself that he strove. . . . HERSCHEL'S instruments were designed to aid vision to the last extent. They were only secondarily for the taking of measures. His efforts were not for a knowledge of the _motions_, but of the _const.i.tution_ and _construction_ of the heavenly bodies."

Besides the stands for his telescopes, which were both ingenious and convenient, HERSCHEL devised many forms of apparatus for facilitating the art of observation. His micrometers for measuring position angles, his lamp micrometer, the method of limiting apertures, and the methods he used for viewing the sun may be mentioned among these.

Points in practical astronomy are considered all through the years of observation. A reference to his original papers will show how numerous, how varied, and how valuable these are. I cannot forbear quoting here the account of a precaution observed during his examination of the belts on _Saturn_ (1794).

It is the most striking example of how fully HERSCHEL realized that the eye of the observer is a material part of the optical apparatus of astronomy. Simple as this principle may appear, it was an absolute novelty in his day.

In making these observations, he says:

"I took care to bend my head so as to receive the picture of the belt in the same direction as I did formerly. This was a precaution that occurred to me, as there was a possibility that the vertical diameter of the retina might be more or less sensitive than the horizontal one."

Astronomers will recognize in this the first suggestion of the processes which have led to important results in the hands of Dr. OTTO STRUVE and others in the comparison of the measures of double stars by different observers, each of whom has a personal habit of observation, which, if not corrected, may affect his results in the way which HERSCHEL was striving to avoid.

_Researches on the Relative Brightness of the Stars: Variable Stars._

No research of HERSCHEL'S was more laborious than the elaborate cla.s.sification of the stars according to their comparative brightness, which he executed during the years 1796 to 1799. It was directly in the line of his main work--to find out the construction of the heavens.

His first paper had been upon the variable star _Mira Ceti_. Here was a sun, shining by its native brightness, which waxed and waned like the moon itself. This star is periodic. It is for a long period invisible to the una.s.sisted eye. Then it can just be seen, and increases in brightness for a little over a month, and attains a maximum brilliancy.

From this it decreases for nearly three months, and after becoming invisible, remains so for five or six months. Its whole period is about 333 days. Are all other stars constant in brightness? The example of _Mira Ceti_ and of other known variables makes this at least doubtful.

But the sun itself may vary for all that we know. It is a simple star like the rest.

This question of variability in general is an important one, then.

It can only be tested by making accurate catalogues of the relative brilliance of stars at various times, and by comparing these. No such general catalogue existed before HERSCHEL'S time, and led by the discrepancies in isolated cases, which he found between his own estimates and those of his predecessors, he made from observation a series of four catalogues, in which were set down the order of sequence of the stars of each constellation.

The method adopted by HERSCHEL was perfectly simple in principle, though most laborious in practice. Suppose any number of stars, A, B, C, D, E, . . . etc., near enough to each other to be well compared. The process consists simply in writing down the names of the stars, A, B, C, etc., in the order of their relative brightness. Thus if for a group of eight stars we have found at one epoch A, B, C, D, E, F, G, H, and if at another time the order was A, B, C, D, F, E, G, H, symptoms of variability are pointed out. Repeated observations, where the same star is found in different sequences, will decide the question. Thus, for the stars visible to the naked eye, we know exactly the state of the sky in HERSCHEL'S day, now nearly a century ago. Any material change cannot escape us. These catalogues have been singularly overlooked by the observers of our generation who have followed this branch of observation, and it was not till 1876 that they received proper attention and a suitable reduction (at the hands of Mr. C. S. PIERCE).

We owe to HERSCHEL the first trustworthy account of the stars visible to the naked eye, and since the date of his labors (about 1800) we have similar views published by ARGELANDER (1839), HEIS (1848), ARGELANDER and SCHoNFELD (1857), GOULD (1860 and 1872), and HOUZEAU (1875). Thus his labors have been well followed up.

In the prosecution of this work HERSCHEL found stars whose light was progressively diminishing, others which regularly increased, one star whose light periodically varies (_[alpha] Herculis_), and at least one star (55 _Herculis_) which has utterly disappeared. On October 10, 1781, and April 11, 1782, he observed this latter star, but in May, 1791, it had totally vanished. There was no trace remaining.

The discovery of the variability of _[alpha] Herculis_ was a more important one than would at first sight appear. Up to that time the only variable stars known were seven in number. Their periods were four hundred and ninety-four, four hundred and four, three hundred and thirty-four, seven, six, five, and three days. These periods seemed to fall into two groups, one of from three hundred to five hundred days, the other comparatively much shorter, of three to seven days.

_[alpha] Herculis_ came to occupy the middle place between these groups, its period being about sixty days.

The cause of these strange and regular variations of brightness was supposed by HERSCHEL to be the rotation of the star bodily on an axis, by which revolution different parts of its surface, of different brilliancy, were successively and periodically presented to us. This explanation it might have been difficult to receive, when the periods of the known variables were so markedly various in length. His own discovery came to bridge over the interval, and quite confirmed him in his belief. He returned to the subject of the revolution of stars about their axes again and again, and connected it with the revolution of satellites.

He found that the satellites of _Jupiter_ and one of _Saturn's_ periodically changed in brightness, and by quite simple means showed that their periods of rotation were at least approximately the same as their periods of revolution about their primaries. In this case, as in every other, he considered a discovery in each and every one of its possible bearings. There are no instances where he has singularly overlooked the consequences of his observations.

_Researches on Double Stars._

The double stars were the subject of HERSCHEL'S earliest and of his latest papers. In 1782 he published his "_Catalogue of Double Stars_,"

and his last published memoir (1822) was on the same subject.

The question of determining the parallax of stars first brought HERSCHEL to the discovery of double stars. If two stars, A and B, appear very close together, and if, in reality, the star B is very many times more distant from the earth than A, although seen along the same line of sight, then the revolution of the earth in its...o...b..t will produce changes in the relative situation of A and B, and, in fact, B will describe a small orbit about A, due to this revolution. This idea had been proposed by GALILEO, and measures on this plan had been made by LONG, with negative results. But HERSCHEL, in reviewing their work, declares that the stars chosen by LONG were not suitable to the purpose.

It is necessary, among other things, to the success of this method, that it should be certain that the star B is really very much more distant than the star A. The only general test of the distance of stars is their brilliancy, and HERSCHEL decided to use only stars for this research which had two components very greatly different in brightness. A must be very bright (and presumably near to us), and B must be very close to A, and very faint (and thus, presumably, very distant).

It was in the search for such pairs of stars that the _Catalogue of Double Stars_ (1782) was formed. HERSCHEL'S first idea of a double star made such pairs as he found, to consist of two stars _accidentally_ near to each other. A was near to us, and appeared projected in a certain place on the celestial sphere. B was many times more distant, but, by chance, was seen along the same line, and made with A an _optical_ double. If the two stars were at the same distance from the earth, if they made part of the same physical system, if one revolved around the other, then this method of gaining a knowledge of their distance failed.

Even in his first memoir on the subject, a surmise that this latter state might occur in some cases, was expressed by HERSCHEL. The notes on some of the pairs declare that a motion of one of them was suspected.

But this motion might be truly orbital--of one star about the other as a centre--or it might simply be that one star was moving by its own _proper_ motion, and leaving the other behind. It was best to wait and see. The first Catalogue of Double Stars contained two hundred and three instances of such a.s.sociations. These were observed from time to time, and new pairs discovered. The paper of MICh.e.l.l, "An Inquiry into the probable Parallax and Magnitude of the Fixed Stars, from the Quant.i.ty of Light which they Afford, and the Particular Circ.u.mstances of their Situation" (1767), was read and pondered. By 1802 HERSCHEL had become certain that there existed in the heavens real pairs of stars, both at the same distance from the earth, which were physically connected with each other. The arguments of MICh.e.l.l have been applied by BESSEL to the case of one of HERSCHEL'S double stars, in much the same order in which the argument ran in HERSCHEL'S own mind, as follows:

The star _Castor_ (_[alpha] Geminorum_) is a double star, where A is of the second, and B of the fourth, magnitude. To the naked eye these two appear as one star. With a telescope this is seen to be two stars, some 5" apart. In the whole sky there are not above fifty such stars as the brighter of the two, and about four hundred of the brilliancy of B. These fifty and four hundred stars are scattered over the vault of heaven, almost at random. No law has yet been traced by which we can say that here or here there shall be a bright star like A, or a fainter one like B. In general the distribution appears to be fortuitous. How then can we account for one of the four hundred stars like B placed so close to one of the fifty like A?

The chances are over four hundred thousand to one that the a.s.sociation in position is not accidental. This argument becomes overwhelming when the same a.s.sociation is found in many other cases. There were two hundred and three doubles in the Catalogue of 1782 alone, and many thousands are now known.

By a process like this, HERSCHEL reached his grand discovery of true binary systems, where one sun revolves about another. For he saw that if the two stars are near together in s.p.a.ce, they could not stand still in face of each other, but that they must revolve in true orbits. Here was the discovery which came to take the place of the detection of the parallaxes of the fixed stars.

He had failed in one research, but he was led to grand conclusions.

Was the force that these distant pairs of suns obeyed, the force of gravitation? This he could not settle, but his successors have done so.

It was not till about 1827 that SAVARY, of the Paris Observatory, showed that one of HERSCHEL'S doubles was subjected to the law of gravitation, and thus extended the power of this law from our system to the universe at large. HERSCHEL himself lived to see some of his double stars perform half a revolution.

Of HERSCHEL'S discoveries, ARAGO thinks this has "le plus d'avenir."

It may well be so. The laws which govern our solar system have been extended, through his researches, to regions of unknown distance. The binary stars will afford the largest field for research into the laws which govern them, and together with the cl.u.s.ters and groups, they will give a firm basis by which to study the distribution of stars in general, since here we have the great advantage of knowing, if not the real distance of the two stars from the earth, at least that this distance is alike for both.

_Researches on Planets and Satellites._

After HERSCHEL'S first publication on the mountains of the Moon (1780), our satellite appears to have occupied him but little. The observation of volcanoes (1787) and of a lunar eclipse are his only published ones.

The planets _Mercury_, _Venus_, _Mars_, and _Jupiter_, although they were often studied, were not the subjects of his more important memoirs.

The planet _Saturn_, on the contrary, seems never to have been lost sight of from the time of his first view of it in 1772.

The field of discovery always appears to be completely occupied until the advent of a great man, who, even by his way of putting old and familiar facts, shows the paths along which discoveries must come, if at all. This faculty comes from profound reflection on the nature of the subject itself, from a sort of trans.m.u.ting power which changes the words of the books into the things of reality. HERSCHEL'S paper on _Saturn_, in 1790, is an admirable example of this.

HERSCHEL'S observations on _Saturn_ began in 1772. From 1790 to 1808 he published six memoirs on the figure, the ring, and the satellites of this planet. The spheroidal shape of the ball was first discovered by him, and we owe much of our certain knowledge of the const.i.tution of the rings to his work. The sixth and seventh satellites, _Mimas_ and _Enceladus_, were discovered by him in 1789. The periods of rotation of the ball and of the ring were also fixed. In his conclusions as to the real figure of the rings, there is a degree of scientific caution which is truly remarkable, and which to-day seems almost excessive.

In his paper of 1792, HERSCHEL shows that the most distant satellite of _Saturn_--_j.a.petus_--turns once on its axis in each revolution about its primary, just as our moon does. He says of this:

"I cannot help reflecting with some pleasure on the discovery of an a.n.a.logy which shows that a certain uniform plan is carried on among the secondary planets of our solar system; and we may conjecture that probably most of the satellites are governed by the same law; especially if it be founded on such a construction of their figure as makes them more ponderous towards their primary planets."

I believe the last suggestion to have been the first statement of the possible arrangement of matter in satellites, which was afterwards so forcibly maintained by HANSEN in his theory of the moon. HANSEN'S researches show the consequences of such an arrangement, although they do not prove its existence.

It should be recorded that the explanation which is to-day received of the belts and bands upon _Jupiter_, is, I believe, first found in HERSCHEL'S memoir on _Venus_ (1793). His memoir of 1797, on the changeable brightness of the satellites of _Jupiter_, has already been referred to. The times of the rotation of the satellites on their axes was first determined by HERSCHEL from these observations, which also contain accounts of the curious, and as yet unexplained, phenomena attending their appearances on the disc of the planet.

HERSCHEL discovered in January, 1787, the two brighter satellites of _Ura.n.u.s_, now called _Oberon_ and _t.i.tania_. They are among the faintest objects in the solar system. A later discussion of all his observations led him to the belief that there were four more, and he gives his observations and computations in full. He says that of the existence of additional satellites he has no doubt. Of these four, three were exterior to the most distant satellite _Oberon_, the other was "interior" to _t.i.tania_.

It was not until 1834 that even _Oberon_ and _t.i.tania_ were again observed (by Sir JOHN HERSCHEL) with a telescope of twenty feet, similar to that which had discovered them, and not until 1847 was the true state of this system known, when Mr. La.s.sELL discovered _Ariel_ and _Umbriel_, two satellites interior to _t.i.tania_, neither of which was HERSCHEL'S "interior" satellite. In 1848 and later years Mr. La.s.sELL, by the aid of telescopes constructed by himself, fully settled the fact that only four satellites of this planet existed. In 1874 I examined the observations of HERSCHEL on his supposed "interior" satellite, thinking that it might be possible that among the very few glimpses of it which he recorded, some might have belonged to _Ariel_ and some to _Umbriel_, and that by combining rare and almost accidental observations of two satellites which really existed, he had come to announce the existence of an "interior" satellite which had no existence in fact. Such I believe to be the case. In 1801, April 17, HERSCHEL describes an interior satellite in the position angle 189, distant 18" from the planet. At that instant _Umbriel_, one of Mr. La.s.sELL'S satellites, was in the position 191, and distant 21" from _Ura.n.u.s_, in the most favorable position for seeing it. The observation of 1794, March 27, _may_ belong to _Ariel_. At the best the investigation is of pa.s.sing interest only, and has nothing to do with the question of the discovery of the satellites. HERSCHEL discovered the two brighter ones, and it was only sixty years later that they were properly re-observed by Mr.

La.s.sELL, who has the great honor of having added as many more, and who first settled the vexed question of satellites _exterior_ to _Oberon_, and this with a reflecting telescope made by himself, which is unequalled by any other of its dimensions.

_Researches on the Nature of the Sun._