"When I resided at Bath I had long been acquainted with the theory of optics and mechanics, and wanted only that experience so necessary in the practical part of these sciences. This I acquired by degrees at that place, where in my leisure hours, by way of amusement, I made several two-foot, five-foot, seven-foot, ten-foot, and twenty-foot Newtonian telescopes, beside others, of the Gregorian form, of eight, twelve, and eighteen inches, and two, three, five, and ten feet focal length. In this way I made not less[Pg 123]than two hundred seven-foot, one hundred and fifty ten-foot, and about eighty twenty-foot mirrors, not to mention the Gregorian telescopes.[32]"The number of stands I invented for these telescopes it would not be easy to assign. . . . In 1781 I began to construct a thirty-foot aërial reflector, and having made a stand for it, I cast the mirror thirty-six inches in diameter. This was cracked in cooling. I cast it a second time, and the furnace I had built in my house broke."
"When I resided at Bath I had long been acquainted with the theory of optics and mechanics, and wanted only that experience so necessary in the practical part of these sciences. This I acquired by degrees at that place, where in my leisure hours, by way of amusement, I made several two-foot, five-foot, seven-foot, ten-foot, and twenty-foot Newtonian telescopes, beside others, of the Gregorian form, of eight, twelve, and eighteen inches, and two, three, five, and ten feet focal length. In this way I made not less[Pg 123]than two hundred seven-foot, one hundred and fifty ten-foot, and about eighty twenty-foot mirrors, not to mention the Gregorian telescopes.[32]
"The number of stands I invented for these telescopes it would not be easy to assign. . . . In 1781 I began to construct a thirty-foot aërial reflector, and having made a stand for it, I cast the mirror thirty-six inches in diameter. This was cracked in cooling. I cast it a second time, and the furnace I had built in my house broke."
Soon after, the Georgian planet was discovered, and this interrupted the work for a time.
"In the year 1783 I finished a very good twenty-foot reflector with a large aperture, and mounted it upon the plan of my present telescope. After two years' observation with it, the great advantage of such apertures appeared so clearly to me that I recurred to my former intention of increasing them still further; and being now sufficiently provided with experience in the work which I wished to undertake, the President of the Royal Society, who is always ready to promote useful undertakings, had the goodness to lay my design before the king. His Majesty was graciously pleased to approve of it, and with his usual liberality to support it with his royal[Pg 124]bounty."In consequence of this arrangement I began to construct the forty-foot telescope about the latter end of 1785.[33]The woodwork of the stand and machines for giving the required motions to the instrument were immediately put in hand. In the whole of the apparatus none but common workmen were employed, for I made drawings of every part of it, by which it was easy to execute the work, as I constantly inspected and directed every person's labor; though sometimes there were not less than forty different workmen employed at the same time. While the stand of the telescope was preparing, I also began the construction of the great mirror, of which I inspected the casting, grinding, and polishing, and the work was in this manner carried on with no other interruption than that occasioned by the removal of all the apparatus and materials from where I then lived, to my present situation at Slough.[Pg 125]"Here, soon after my arrival, I began to lay the foundation upon which by degrees the whole structure was raised as it now stands, and the speculum being highly polished and put into the tube, I had the first view through it on February 19, 1787. I do not, however, date the completing of the instrument till much later. For the first speculum, by a mismanagement of the person who cast it, came out thinner on the centre of the back than was intended, and on account of its weakness would not permit a good figure to be given to it."A second mirror was cast January 26, 1788, but it cracked in cooling. February 16 we recast it, and it proved to be of a proper degree of strength. October 24 it was brought to a pretty good figure and polish, and I observed the planetSaturnwith it. But not being satisfied, I continued to work upon it till August 27, 1789, when it was tried upon the fixed stars, and I found it to give a pretty sharp image. Large stars were a little affected with scattered light, owing to many remaining scratches on the mirror. August the 28th, 1789, having brought the telescope to the parallel ofSaturn, I discovered asixthsatellite of that planet, and also saw the spots uponSaturnbetter than I had ever seen them before, so that I may date the finishing of the forty-foot telescope from that time."
"In the year 1783 I finished a very good twenty-foot reflector with a large aperture, and mounted it upon the plan of my present telescope. After two years' observation with it, the great advantage of such apertures appeared so clearly to me that I recurred to my former intention of increasing them still further; and being now sufficiently provided with experience in the work which I wished to undertake, the President of the Royal Society, who is always ready to promote useful undertakings, had the goodness to lay my design before the king. His Majesty was graciously pleased to approve of it, and with his usual liberality to support it with his royal[Pg 124]bounty.
"In consequence of this arrangement I began to construct the forty-foot telescope about the latter end of 1785.[33]The woodwork of the stand and machines for giving the required motions to the instrument were immediately put in hand. In the whole of the apparatus none but common workmen were employed, for I made drawings of every part of it, by which it was easy to execute the work, as I constantly inspected and directed every person's labor; though sometimes there were not less than forty different workmen employed at the same time. While the stand of the telescope was preparing, I also began the construction of the great mirror, of which I inspected the casting, grinding, and polishing, and the work was in this manner carried on with no other interruption than that occasioned by the removal of all the apparatus and materials from where I then lived, to my present situation at Slough.
[Pg 125]"Here, soon after my arrival, I began to lay the foundation upon which by degrees the whole structure was raised as it now stands, and the speculum being highly polished and put into the tube, I had the first view through it on February 19, 1787. I do not, however, date the completing of the instrument till much later. For the first speculum, by a mismanagement of the person who cast it, came out thinner on the centre of the back than was intended, and on account of its weakness would not permit a good figure to be given to it.
"A second mirror was cast January 26, 1788, but it cracked in cooling. February 16 we recast it, and it proved to be of a proper degree of strength. October 24 it was brought to a pretty good figure and polish, and I observed the planetSaturnwith it. But not being satisfied, I continued to work upon it till August 27, 1789, when it was tried upon the fixed stars, and I found it to give a pretty sharp image. Large stars were a little affected with scattered light, owing to many remaining scratches on the mirror. August the 28th, 1789, having brought the telescope to the parallel ofSaturn, I discovered asixthsatellite of that planet, and also saw the spots uponSaturnbetter than I had ever seen them before, so that I may date the finishing of the forty-foot telescope from that time."
Another satellite ofSaturnwas discovered with the forty-foot on the 17th of September (1789). It was used for various observationsso late as 1811. On January 19, of that year,Herschelobserved the nebula ofOrionwith it. This was one of his last observations.
The final disposition of the telescope is told in the following extract from a letter of SirJohn Herschel'sto Mr.Weld, Secretary of the Royal Society:
Collingwood,March 13, 1847.
Collingwood,March 13, 1847.
. . . "In reply to your queries, respecting the forty-foot reflecting telescope constructed by my father, I have to state that KingGeorge III.munificently defrayed theentirecost of that instrument (including, of course, all preparatory cost in the nature of construction of tools, and of the apparatus for casting, grinding, and figuring the reflectors, of which two were constructed), at a total cost of £4,000. The woodwork of the telescope being so far decayed as to be dangerous, in the year 1839 I pulled it down, and piers were erected on which the tube was placed,thatbeing of iron and so well preserved, that, although not more than one-twentieth of an inch thick, when in the horizontal position it sustained within it all my family, and continues to sustain inclosed within it, to this day, not only the heavier of the two reflectors, but also all the more important portions of the machinery. . . . The other mirror and the rest of the polishing apparatus are on the[Pg 127]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.". . .
. . . "In reply to your queries, respecting the forty-foot reflecting telescope constructed by my father, I have to state that KingGeorge III.munificently defrayed theentirecost of that instrument (including, of course, all preparatory cost in the nature of construction of tools, and of the apparatus for casting, grinding, and figuring the reflectors, of which two were constructed), at a total cost of £4,000. The woodwork of the telescope being so far decayed as to be dangerous, in the year 1839 I pulled it down, and piers were erected on which the tube was placed,thatbeing of iron and so well preserved, that, although not more than one-twentieth of an inch thick, when in the horizontal position it sustained within it all my family, and continues to sustain inclosed within it, to this day, not only the heavier of the two reflectors, but also all the more important portions of the machinery. . . . The other mirror and the rest of the polishing apparatus are on the[Pg 127]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 SirJohn, was read, the machinery put into its present position, and the tube sealed.
The memoir on the forty-foot telescope shows throughout thatHerschel'sprime 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 thelimit of vision. AsBesselhas 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[Pg 128]more powerful one penetrates deeper into space, 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 ofHerschel'stelescopes will have observed this. ButHerschelwas not content with the bare fact, but strove ever to knowhow fara telescope of a certain construction and size could penetrate, compared with the naked and unassisted 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 space itself that he strove. . . .Herschel'sinstruments 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 themotions, but of theconstitutionandconstructionof the heavenly bodies."
"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[Pg 128]more powerful one penetrates deeper into space, 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 ofHerschel'stelescopes will have observed this. ButHerschelwas not content with the bare fact, but strove ever to knowhow fara telescope of a certain construction and size could penetrate, compared with the naked and unassisted 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 space itself that he strove. . . .Herschel'sinstruments 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 themotions, but of theconstitutionandconstructionof the heavenly bodies."
Besides the stands for his telescopes, which were both ingenious and convenient,Herscheldevised 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 onSaturn(1794).
It is the most striking example of how fullyHerschelrealized 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."
"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 Struveand 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, mayaffect his results in the way whichHerschelwas striving to avoid.
Researches on the Relative Brightness of the Stars: Variable Stars.
No research ofHerschel'swas more laborious than the elaborate classification 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 starMira 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 unassisted 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 ofMira Cetiand 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 beforeHerschel'stime, 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 byHerschelwas 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 oneepoch 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 inHerschel'sday, 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 toHerschelthe 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 byArgelander(1839),Heis(1848),ArgelanderandSchönfeld(1857),Gould(1860 and 1872), andHouzeau(1875). Thus his labors have been well followed up.
In the prosecution of this workHerschelfound stars whose light was progressivelydiminishing, others which regularly increased, one star whose light periodically varies (α Herculis), and at least one star (55Herculis) 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 ofα Herculiswas 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.α Herculiscame 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 byHerschelto be the rotation of the star bodily on an axis, by which revolution different partsof 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 ofJupiterand one ofSaturn'speriodically 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 ofHerschel'searliest and of his latest papers. In1782 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 broughtHerschelto 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 orbit 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 byGalileo, and measures on this plan had been made byLong, with negative results. ButHerschel, in reviewing their work, declares that the stars chosen byLongwere 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, andHerscheldecided to use onlystars 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 theCatalogue of Double Stars(1782) was formed.Herschel'sfirst idea of a double star made such pairs as he found, to consist of two starsaccidentallynear 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 anopticaldouble. 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 byHerschel. The notes on some of the pairs declare that a motion of one of them was suspected. But this motion might be truly orbital—ofone star about the other as a centre—or it might simply be that one star was moving by its ownpropermotion, 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 associations. These were observed from time to time, and new pairs discovered. The paper ofMichell, "An Inquiry into the probable Parallax and Magnitude of the Fixed Stars, from the Quantity of Light which they Afford, and the Particular Circumstances of their Situation" (1767), was read and pondered. By 1802Herschelhad 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 ofMichellhave been applied byBesselto the case of one ofHerschel'sdouble stars, in much the same order in which the argument ran inHerschel'sown mind, as follows:
The starCastor (α Geminorum)is a double star, where A is of the second, and B of the fourth, magnitude. To the naked eyethese 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 association in position is not accidental. This argument becomes overwhelming when the same association 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,Herschelreached his grand discovery of true binary systems, where one sun revolves about another. Forhe saw that if the two stars are near together in space, 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 thatSavary, of the Paris Observatory, showed that one ofHerschel'sdoubles was subjected to the law of gravitation, and thus extended the power of this law from our system to the universe at large.Herschelhimself lived to see some of his double stars perform half a revolution.
OfHerschel'sdiscoveries,Aragothinks 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 togetherwith the clusters 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.
AfterHerschel'sfirst 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 planetsMercury,Venus,Mars, andJupiter, although they were often studied, were not the subjects of his more important memoirs. The planetSaturn, 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. Thisfaculty comes from profound reflection on the nature of the subject itself, from a sort of transmuting power which changes the words of the books into the things of reality.Herschel'spaper onSaturn, in 1790, is an admirable example of this.
Herschel'sobservations onSaturnbegan 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 constitution of the rings to his work. The sixth and seventh satellites,MimasandEnceladus, 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,Herschelshows that the most distant satellite ofSaturn—Japetus—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 analogy 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 cannot help reflecting with some pleasure on the discovery of an analogy 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 byHansenin his theory of the moon.Hansen'sresearches 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 uponJupiter, is, I believe, first found inHerschel'smemoir onVenus(1793). His memoir of 1797, on the changeable brightness of the satellites ofJupiter, has already been referred to. The times of the rotation of the satellites on their axes was first determined byHerschelfrom these observations, whichalso contain accounts of the curious, and as yet unexplained, phenomena attending their appearances on the disc of the planet.
Herscheldiscovered in January, 1787, the two brighter satellites ofUranus, now calledOberonandTitania. 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 satelliteOberon, the other was "interior" toTitania.
It was not until 1834 that evenOberonandTitaniawere again observed (by SirJohn 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.LasselldiscoveredArielandUmbriel, two satellites interior toTitania, neither of which wasHerschel's"interior" satellite. In 1848 and later years Mr.Lassell, by the aid of telescopes constructedby himself, fully settled the fact that only four satellites of this planet existed. In 1874 I examined the observations ofHerschelon 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 toArieland some toUmbriel, 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,Herscheldescribes an interior satellite in the position angle 189°, distant 18″ from the planet. At that instantUmbriel, one of Mr.Lassell'ssatellites, was in the position 191°, and distant 21″ fromUranus, in the most favorable position for seeing it. The observation of 1794, March 27,maybelong toAriel. At the best the investigation is of passing interest only, and has nothing to do with the question of the discovery of the satellites.Herscheldiscovered the two brighter ones, and it wasonly sixty years later that they were properly re-observed by Mr.Lassell, who has the great honor of having added as many more, and who first settled the vexed question of satellitesexteriortoOberon, 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.
In the introduction to his paper on theNature and Construction of the Sun and Fixed Stars(1795),Herschelrecounts what was known of the nature of the sun at that time.Newtonhad shown that it was the centre of the system;Galileoand his successors had determined its rotation, the place of its equator, its real diameter, magnitude, density, distance, and the force of gravity on its surface. He says:
"I should not wonder if, considering all this, we were induced to think that nothing remained to be added; and yet we are still very ignorant in regard to the internal construction of the sun." "The[Pg 146]spots have been supposed to be solid bodies, the smoke of volcanoes, the scum floating on an ocean of fluid matter, clouds, opaque masses, and to be many other things." "The sun itself has been called a globe of fire, though, perhaps, metaphorically." "It is time now to profit by the observations we are in possession of. I have availed myself of the labors of preceding astronomers, but have been induced thereto by my own actual observation of the solar phenomena."
"I should not wonder if, considering all this, we were induced to think that nothing remained to be added; and yet we are still very ignorant in regard to the internal construction of the sun." "The[Pg 146]spots have been supposed to be solid bodies, the smoke of volcanoes, the scum floating on an ocean of fluid matter, clouds, opaque masses, and to be many other things." "The sun itself has been called a globe of fire, though, perhaps, metaphorically." "It is time now to profit by the observations we are in possession of. I have availed myself of the labors of preceding astronomers, but have been induced thereto by my own actual observation of the solar phenomena."
Herschelthen refers to the theories advanced by his friend, Prof.Wilson, of Glasgow, in 1774.Wilsonmaintained that the spots were depressions below the sun's atmosphere, vast hollows as it were, at the bases of which the true surface of the sun could be seen.
The essence of his theory was the existence of two different kinds of matter in the sun: one solid and non-luminous—the nucleus—the other gaseous and incandescent—the atmosphere. Vacant places in the atmosphere, however caused, would show the black surface of the solid mass below. These were the spots. No explanation could be given of thefaculæ, bright streaks, which appearon the sun's surface from time to time; but his theory accounted for the existence of the blacknucleiof the spots, and for the existence of thepenumbræabout these. The penumbra of a spot was formed by the thinner parts of the atmosphere about the vacancy which surrounded the nucleus.
This theory ofWilson'swas adopted byHerschelas a basis for his own, and he brought numerous observations to confirm it, in the modified shape which he gave to it.
According toHerschel, the sun consisted of three essentially different parts. First, there was a solid nucleus, non-luminous, cool, and even capable of being inhabited. Second, above this was an atmosphere proper; and, lastly, outside of this was a layer in which floated the clouds, or bodies which gave to the solar surface its intense brilliancy:
"According to my theory, a dark spot in the sun is a place in its atmosphere which happens to be free from luminous decompositions" above it.
"According to my theory, a dark spot in the sun is a place in its atmosphere which happens to be free from luminous decompositions" above it.
The two atmospheric layers, which will be of varying thickness about a spot, willaccount for all the shades of darkness seen in the penumbra. Ascending currents from the solar surface will elevate certain regions, and may increase the solar activity near by, and will thus give rise to faculæ, whichHerschelshows to be elevated above the general surface. It will not be necessary to give a further account of this theory. The data in the possession of the modern theorist is a thousand-fold that to be derived fromHerschel'sobservations, and, while the subject of the internal construction of the sun is to-day unsettled, we know that many important, even fundamental, portions of his theory are untenable. A remark of his should be recorded, however, as it has played a great part in such theories:
"That the emission of light must waste the sun, is not a difficulty that can be opposed to our hypothesis. Many of the operations of Nature are carried on in her great laboratory which we cannot comprehend. Perhaps the many telescopic comets may restore to the sun what is lost by the emission of light."
"That the emission of light must waste the sun, is not a difficulty that can be opposed to our hypothesis. Many of the operations of Nature are carried on in her great laboratory which we cannot comprehend. Perhaps the many telescopic comets may restore to the sun what is lost by the emission of light."
Arguments in favor of the habitability of both sun and moon are contained in thispaper; but they rest more on a metaphysical than a scientific basis, and are to-day justly forgotten.
Researches on the Motion of the Sun and of the Solar System in Space.
In 1782Herschelwrites, in regard to some of his discoveries of double stars:
"These may serve another very important end. I will just mention it, though it is foreign to my present purpose. Several stars of the first magnitude have been observed or suspected to have a proper motion; hence we may surmise that our sun, with all its planets and comets, may also have a motion towards some particular point of the heavens. . . . If this surmise should have any foundation, it will show itself in a series of some years in a kind of systematical parallax, or change, due to the motion of the whole solar system."
"These may serve another very important end. I will just mention it, though it is foreign to my present purpose. Several stars of the first magnitude have been observed or suspected to have a proper motion; hence we may surmise that our sun, with all its planets and comets, may also have a motion towards some particular point of the heavens. . . . If this surmise should have any foundation, it will show itself in a series of some years in a kind of systematical parallax, or change, due to the motion of the whole solar system."
In 1783 he published his paperOn the Proper Motion of the Solar System, which contained the proofs of his surmises of a year before. That certain of the stars had in fact apropermotion had been well established by the astronomers of the eighteenth century.After all allowances had been made for the effects of precession and other displacements of a star's position which were produced by motions of the earth, it was found that there were still small outstanding differences which must be due to the motion of the star itself—its proper motion. The quantity of this motion was not well known for any star whenHerschel'sresearches began. Before they were concluded, however,Maskelynehad deduced the proper motions of thirty-six stars—the fundamental stars, so called—which included in their numberSirius,Procyon,Arcturus, and generally the brightest stars.
It isà priorievident that stars, in general, must have proper motions, when once we admit the universality of gravitation. That any fixed star should be entirely at rest would require that the attractions on all sides of it should be exactly balanced. Any change in the position of this star would break up this balance, and thus, in general, it follows that stars must be in motion, since all of them cannot occupy such a critical position as has to be assumed. If but one fixed star is inmotion, this affects all the rest, and we cannot doubt but that every star, our sun included, is in motion by an amount which varies from small to great. If the sun alone had a motion, and the other stars were at rest, the consequence of this would be that all the fixed stars would appear to be retreatingen massefrom that point in the sky towards which we were moving. Those nearest us would move more rapidly, those more distant less so. And in the same way, the stars from which the solar system was receding would seem to be approaching each other. If the stars, instead of being quite at rest, as just supposed, had motions proper to themselves, then we should have a double complexity. They would still appear to an observer in the solar system to have motions, and part of these motions would be truly proper to the stars, and part would be due to the advance of the sun itself in space.
Observations can show us only theresultantof these two motions. It is for reasoning to separate this resultant into its two components. At first the question is to determinewhether the results of observation indicate any solar motion at all. If there is none, the proper motions of stars will be directed along all possible lines. If the sun does truly move, then there will be a general agreement in the resultant motions of the stars near the ends of the line along which it moves, while those at the sides, so to speak, will show comparatively less systematic effect. It is as if one were riding in the rear of a railway train and watching the rails over which it has just passed. As we recede from any point, the rails at that point seem to come nearer and nearer together.
If we were passing through a forest, we should see the trunks of the trees from which we were going apparently come nearer and nearer together, while those on the sides of us would remain at their constant distance, and those in front would grow further and further apart.
These phenomena, which occur in a case where we are sensible of our own motion, serve to show how we may deduce a motion,otherwise unknown, from the appearances which are presented by the stars in space.
In this way, acting upon suggestions which had been thrown out previously to his own time byLambert,Mayer, andBradley,Herscheldemonstrated that the sun, together with all its system, was moving through space in an unknown and majestic orbit of its own. The centre round which this motion is directed cannot yet be assigned. We can only know the point in the heavens towards which our course is directed—"the apex of solar motion."
By a study of the proper motions assigned byMaskelyneto the brighter stars,Herschelwas able to define the position of the solar apex with an astonishing degree of accuracy. His calculations have been several times repeated with the advantage of modern analytical methods, and of the hundred-fold material now at our disposition, but nothing essential has been added to his results of 1805, which were based upon such scanty data; and his paper of 1782 contains the announcement of the discovery itself.
His second paper on theDirectionandVelocityof the solar system (1805) is the best example that can possibly be given of his marvellous skill in reaching the heart of a matter, and it may be the one in which his philosophical powers appear in their highest exercise. For sustained reflection and high philosophic thought it is to be ranked with the researches ofNewtonin thePrincipia.
Researches on the Construction of the Heavens.
Herschel'spapers on the Construction of the Heavens, as he named it, extended over his whole scientific life. By this he specially means the method according to which the stars, the clusters, the nebulæ, are spread through the regions of space, the causes that have led to this distribution, and the laws to which it is subjected.
No single astronomical fact is unimportant in the light which it may throw on the scheme of the whole, and each fact is to be considered in this light. As an instance: his discovery of the variable starα Herculis,which has a period of sixty days, was valuable in itself as adding one more to the number of those strange suns whose light is now brighter, now fainter, in a regular and periodic order. But the chief value of the discovery was that now we had an instance of a periodic star which went through all its phases in sixty days, and connected, as it were, the stars of short periods (three to seven days) with those of very long ones (three hundred to five hundred days), which two groups had, until then, been the only ones known. In the same way all his researches on the parallaxes of stars were not alone for the discovery of the distance of any one or two single stars, but to gain a unit of celestial measure, by means of which the depths of space might be sounded.
Astronomy inHerschel'sday considered the bodies of the solar system as separated from each other by distances, and as filling a cubical space. The ideas of near and far, of up and down, were preserved, in regard to them, by common astronomical terms. But the vast number of stars seemed to be thought of, asthey appear in fact to exist, lying on the surface of a hollow sphere. The immediate followers ofBradleyused these fixed stars as points of reference by which the motions within the solar system could be determined, or, likeLacailleandLalande, gathered those immense catalogues of their positions which are so indispensable to the science.MichellandHerschelalone, in England, occupied their thoughts with the nature and construction of the heavens—the one in his study, the other through observation.[34]They were concerned with all three of the dimensions of space.
In his memoir of 1784,Herschelsays:
"Hitherto the sidereal heavens have, not inadequately for the purpose designed, been represented by the concave surface of a sphere, in the centre of which the eye of an observer might be[Pg 157]supposed to be placed."It is true the various magnitudes of the fixed stars even then plainly suggested to us, and would have better suited, the idea of an expanded firmament of three dimensions; but the observations upon which I am now going to enter still farther illustrate and enforce the necessity of considering the heavens in this point of view. In future, therefore, we shall look upon those regions into which we may now penetrate by means of such large telescopes, as a naturalist regards a rich extent of ground or chain of mountains containing strata variously inclined and directed, as well as consisting of very different materials. The surface of a globe or map, therefore, will but ill delineate the interior parts of the heavens."
"Hitherto the sidereal heavens have, not inadequately for the purpose designed, been represented by the concave surface of a sphere, in the centre of which the eye of an observer might be[Pg 157]supposed to be placed.
"It is true the various magnitudes of the fixed stars even then plainly suggested to us, and would have better suited, the idea of an expanded firmament of three dimensions; but the observations upon which I am now going to enter still farther illustrate and enforce the necessity of considering the heavens in this point of view. In future, therefore, we shall look upon those regions into which we may now penetrate by means of such large telescopes, as a naturalist regards a rich extent of ground or chain of mountains containing strata variously inclined and directed, as well as consisting of very different materials. The surface of a globe or map, therefore, will but ill delineate the interior parts of the heavens."
Herschel'smethod of study was founded on a mode of observation which he calledstar-gauging. It consisted in pointing a powerful telescope toward various parts of the heavens, and ascertaining by actual count how thick the stars were in each region. His twenty-foot reflector was provided with such an eye-piece that, in looking into it, he saw a portion of the heavens about 15′ in diameter. A circle of this size on the celestial sphere has about one quarter the apparent surface of the sun, or of the full moon. On pointing the telescope in any direction, agreater or less number of stars were visible. These were counted, and the direction in which the telescope pointed was noted. Gauges of this kind were made in all parts of the sky, and the results were tabulated in the order of right ascension.
The following is an extract from the gauges, and gives the average number of stars in each field at the points noted in right ascension and north polar distance:
In this small table, it is plain that a different law of clustering or of distribution obtains in the two regions. Such differences are still more marked, if we compare the extreme cases found byHerschel, as R. A.= 19h41m, N. P. D. = 74° 33′, number of stars per field = 588; and R. A. = 16h10m, N. P. D. = 113° 4′, number of stars = 1.1.
The number of stars in certain portions is very great. For example, in the Milky Way, nearOrion, six fields of view promiscuously taken gave 110, 60, 70, 90, 70, and 74 stars each, or a mean of 79 stars per field. The most vacant space in this neighborhood gave 60 stars. So that asHerschel'ssweeps were two degrees wide in declination, in one hour (15°) there would pass through the field of his telescope 40,000 or more stars. In some of the sweeps this number was as great as 116,000 stars in a quarter of an hour.
WhenHerschelfirst applied his telescope to the Milky Way, he believed that it completely resolved the whole whitish appearance into small stars. This conclusion he subsequently modified. He says: