The wonders of optics/The telescopes

by Fulgence Marion

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CHAPTER IX.

THE TELESCOPES OF GALILEO, GREGORY, NEWTON, HERSCHEL, LORD ROSSE, AND FOUCAULT.

If history has failed to furnish us with the name of the inventor of the microscope, we have very exact information as to the first experimenters upon the powers of the telescope.

"In the archives of the Hague," says Arago, "we find documents, by the aid of which Van Swieten and Moll have come to a decisive conclusion as to the first and true inventor of the telescope."

We read in these documents that a spectacle-maker of Middleburg, named John Lippershey, addressed a petition to the States-General on October 2, 1606, in which he asked leave to take out a patent, which should constitute him the only maker of this instrument, or which should confer upon him an annual pension, on the condition of not manufacturing them for other nations. The petition qualifies the instrument as serving to see distinct objects, as had already been explained to the members of the States-General.

On the 4th of October, 1608, the States-General appointed a deputy from each province to experiment on the new instrument, which was placed on a tower of the palace belonging to the Stadtholder. Huggard says that the first telescopes experimented on were a foot and a half in length.

On the 6th of October, the commission declared the instrument of Lippershey to be useful to the nation, but demanded that it should be made for two eyes instead of one.

On the 9th of December, Lippershey, having announced that he had solved the problem, Van Dorth, Magnus, and Van der Au were ordered to verify the fact, which they did by making a very favourable report on the 11th of the same month. The binocular instrument was therefore found to answer.

In reading the extracts from the archives of the Hague, given by Moll, we may remark with great pleasure the promptitude with which the commissioners of the States-General examined Lippershey's instruments. But their satisfaction soon gave way to displeasure, when they found a large number of opticians making these instruments, and selling them to foreigners, like so much spice from the East Indies. Later on one feels indignant at finding the commissioners of the States-General to be so wanting in proper feeling as to decide that the telescope must be considered imperfect until it could be used with both eyes, without either winking or seeing the reflection of the pupils in the eye-pieces. Consequently, instead of being permitted to expend his talent on perfecting the optical powers of the single telescope, Lippershey saw himself condemned to waste his time upon the double instrument. The States-General finished by giving Lippershey 900 florins; but they refused him a patent, on the ground that it was already notorious that other opticians had commenced the manufacture of similar instruments.

Amongst others who were rivals of Lippershey, we must mention John Adrian Metius, the son of Adrian Metius, of Amsterdam, who discovered that the nearest relation of the circumference of a circle to its diameter was 355 to 113. He addressed a letter to the States-General on the 17th of October, 1608, conceived in the following terms:—

"After two years' labour and thought I have succeeded in making an instrument, by the aid of which objects which are too distant to be visible by the eye, are seen plainly. The one I show, although constructed out of bad materials, and simply as an experiment, is, in the judgment of the Stadtholder and of several other persons, as good as the one lately presented to the States-General by a citizen of Middleburg. I am sure of improving it still further in the course of time, and I beg to ask for a patent by which any person who is not already in possession of this invention will be forbidden, under pain of a heavy fine and confiscation, to make or sell similar instruments for twenty-two years."

The States-General refused to grant the patent in this case also, but enjoined Metius to perfect his instrument, reserving to themselves the power to reward him in the future if they thought fit.

In Italy, Galileo is generally supposed to have discovered independently the method of making a telescope on the principle of the Dutch philosophers, about the beginning of 1609, having received a very imperfect account of these instruments somewhere about that time. It may be remarked that in his letter to the chiefs of the Venetian Republic, giving an account of the properties of these new instruments, Galileo states that, if necessary, they could be made specially for the use of the navy and army belonging to the state. But secrecy was useless, for telescopes were already made and sold in Holland at a cheap rate. Besides, Galileo makes no allusion to the labours of his Dutch predecessors, either in a prior letter handed down to us by Venturi, or in the decree of the Venetian Senate, dated August 5, 1609.

The Italian commentators are in error when they attribute the second discovery of the telescope to the knowledge that Galileo possessed of the laws of refraction, and that it was by deductions therefrom that he was enabled to construct his first instruments.

Huyghens says, in his Treatise on Dioptrics, "I will unhesitatingly place that man above all mortals, who, by the aid of his own reflections and without the aid of accident, first succeeded in constructing a telescope."

"Let us see," says Arago, when speaking on this subject, "if Lippershey and John Adrian Metius were men of unparalleled powers."

Hieronymus Saturnus tells us that an unknown man of genius called upon Lippershey, and ordered from him a number of convex and concave lenses. At the time agreed upon the man returned, and chose two, one convex and the other concave, and, placing them one before his eye and the other at some distance from it, drew them backwards and forwards, without giving any explanation of his manœuvres, paid the optician, and left the place. As soon as he was gone, Lippershey began immediately to imitate the experiments of the stranger, and soon found that distant objects were brought apparently nearer, when the lenses were placed in certain positions. He next fastened them to the ends of a tube, and lost no time in presenting the new instrument to Prince Maurice of Nassau.

According to another version, Lippershey's children were playing in their father's shop, and were looking through two lenses, one convex and the other concave, when they found to their surprise that the vane on the clock-tower of Middleburg Church was greatly magnified and apparently brought nearer. The surprise expressed by the children having awakened the attention of Lippershey, he tried the experiment of fixing the lenses on a piece of board; afterwards he tried it again by fixing them at the ends of two pieces of tube, sliding in each other, and succeeded in making the first telescope on record.

The principal documents from which the above facts touching Lippershey have been extracted, are to be found in a memoir on the subject by Olbers, printed in Schumacher's Astronomical Annual for 1843.

It was said in the time of Galileo that he had in his possession a telescope by the aid of which he could see the birds flying at Fiesole from the window of his palace in Florence. This story does not in the least detract from the merit of the illustrious astronomer, who not only constructed a telescope for himself, but was the first to direct it heavenwards, and that too by purely theoretical researches; for in spite of all the documents adduced above, there is little or no proof that he had ever seen or heard of the Dutchman's telescope. It is only right, therefore, that the instrument constructed on this principle should be called the Galilean telescope. He afterwards increased its power from four to thirty times, beyond which he could not get with the means at his command. With his imperfect instruments Galileo discovered the satellites of Jupiter, the mountains of the moon, and the spots on the sun, and earned for himself the name of Lynceus, who according to the ancients was one of the Argonauts, possessed of the power of seeing through a wall. Towards the end of his life, when the old man was blind, and the Academy of the Lincei treated his hypotheses with disdain, he would laugh sadly at the name bestowed on him, and the obstinate Academy. Fig. 41 (see next page) shows the path of the rays in a Galilean telescope. The object-glass o is double convex, aond the eye-piece o bi-concave. The image is formed between these lenses, and the eye appears to see it at that point. The States-General complained of being obliged to shut one eye when looking through a telescope, but in 1671 a good Capuchin monk, whose name was Cherubino, placed two telescopes together, little thinking that the moderns
Fig. 41.—The Galilean Telescope. would imitate him in that very worldly instrument, the opera-glass.

Everybody has noticed that when objects are close to us they appear larger than when they are at a distance; it accordingly amounts to the same thing whether, in speaking of the power of telescopes, we say they magnify twice, four times, or a hundred times, or that they are brought within half, a quarter, or a hundredth of their distance. Thus there is a telescope at Lord Rosse's Observatory, at Parsonstown in Ireland, which is the finest yet constructed. Its highest magnifying power is 6,000, therefore every object we look at with it is brought within the 6,000th of its distance from us. Looking at the moon, for instance, we know that our satellite is distant some 240,000 miles from us; we have, therefore, only to divide that number by 6,000 to find that by means of this wonderful instrument the moon is brought within 40 miles of the earth. This statement, however, is not strictly true, for it supposes the whole of the apparatus used to be theoretically perfect.

Kepler, whose great name is now-a-days always associated with that of Galileo, but who during their life-time was somewhat his rival, substituted for the single lens forming the eye-piece a combination consisting of two convex lenses, in order to obtain a larger field for observation than that given by the single bi-concave. This combination is commonly known as the astronomical eye-piece. It reverses the object looked at, but for astronomical purposes this defect is of no consequence.

Fig. 42.—The Astronomical Telescope.

The instrument shown in the above figure represents an astronomical telescope reduced to its simplest form.

Fixed parallel to the axis of the larger telescope is the finder, a small telescope of low power and large field, used for finding celestial objects not easily visible to the naked eye. It is so arranged, that when the object is found and carried to its centre, it is also in the centre of the field of the larger instrument. The handle and the two toothed wheels serve to raise or lower the telescope, which is movable on the horizontal axis, which supports it in front, so that it may be directed to any part of the heavens the observer may desire.

The following figure shows the arrangement of the lenses, and the path of the rays through them, in telescopes of this form.

Fig. 43.—Section of an Astronomical Telescope.

The convex lens which serves as an object-glass, gives at a b a reversed image of the star A B. The small convex lens which acts the part of an eye-piece, enlarged this reversed image without changing its position, and causes it to be seen in the line A' B'. This eye-piece is fixed at the extremity of a tube, which is smaller than that containing the object-glass, and slides easily backwards and forwards from the spot where the image a b is found. The latter is an indispensable condition, for it is rare to meet two persons whose eyes are of the same focus; besides, the image a b will fall at a different spot for objects at different distances: thus, if you are looking at the moon, and suddenly turn the instrument on to a distant nebula, you will find that the eye-piece requires adjusting. In showing ordinary observers an object in the telescope, it is well to insist on their moving the eye-piece backwards and forwards until distinct vision is obtained, for it often happens that people will say they see an object quite distinctly, when it is in reality misty, and will generally refuse to allow the focus to be altered. It is very singular how human vanity or complaisance will step in when some persons are looking through a telescope. They seem to think that there is some disgrace or rudeness involved in their not being able to see what their predecessors at the instrument have seen. Poor John Leech leaves us an amusing instance of this in a comic cut inserted in one of the early numbers of our old friend Punch. A gentleman is endeavouring to show a lady a distant steamboat through a telescope, but she has it accidentally pointed at two swans that are swimming on the margin of the lake below; consequently when he asks her if she sees the steamer, she replies that "she sees it most distinctly, and there are two of them," a pretty good proof that the instrument was not only pointed at the wrong object, but was out of focus as well.

In constructing a telescope similar to the one described above, the object-glass ought to be of considerable diameter and of long focus; the eye-piece, on the contrary, should be comparatively small and of short focus. A little consideration will show the reason of this. An object-glass of long focus will form a large image at the point a b, and the eye-piece of short focus will magnify this image more than another lens of less convexity. It is, however, on the size, length of focus, and perfection of workmanship of the objective that the excellence of the telescope depends; large object-glasses are consequently rare, and are only to be found in observatories of the first class. The object-glass of the large telescope at the Observatory at Paris is nearly fifteen inches in diameter, and the highest magnifying power capable of being employed with it is 3,000. The Observatory of Pulkowa, near St. Petersburg, possesses a similar instrument, and the Observatory at Chicago, United States, a still larger one, measuring between eighteen and nineteen inches in diameter. But the largest of all is an objective in the possession of Mr. Buckingham, an amateur astronomer, who has an observatory near London, which is twenty inches in diameter, and twenty-eight feet in focal length.

The eye-pieces of astronomical telescopes are of different powers, and are changed according to the class of object to be observed. Thus, in taking a general view of the moon, a low power would be used. If you wished to examine any particular mountain, you would raise the magnifying power by inserting a stronger eye-piece. The power used also depends on the state of the atmosphere. For instance, on warm evenings, when the air is charged with moisture, the tremulousness of the atmosphere is so great, that it is often only possible to use the very lowest power. By combining four convex lenses together, we obtain what is called a terrestrial or erecting eye-piece, which has the property of re-reversing the image formed by the objective. The eye-pieces of all telescopes for use on land or at sea are made on this principle. The same effect may be obtained, as we have already shown in fig. 41, by using a concave lens, but in this the field of view is much diminished.

Hitherto we have only spoken of refracting telescopes or those instruments provided with a convex object-glass, to collect and refract the rays of light given off by the object we are desirous of examining; but there is another and very important class of instruments, in which the object-glass is replaced by a reflecting mirror. The first reflecting telescope was invented by Dr. Gregory, an English philosopher, about 1650. It consisted of a brass tube, at the lower extremity of which was fixed a concave mirror made of metal, and provided with a hole in its centre for the insertion of the small tube containing the eye-glass. Towards the other end of the telescope was placed a second and smaller mirror, which reflected the image formed by the large mirror, through the eye-piece to the eye. The following figure will show the path of the rays in the Gregorian telescope.

Fig. 44.—Section of the Gregorian Telescope.

The rays A B, proceeding from the object at which the instrument is pointed, are first reflected from the surface of the principal mirror M M on to the small mirror m, whence they proceed to form a magnified image at a b, which is then again enlarged by the eye-piece appearing to the eye as if placed at A' B'. The focus in the Gregorian is altered, not by sliding the eye-piece backwards and forwards but by moving the mirror m, which is provided with a long screw, to which is attached a handle. At first sight a reflecting tele-scope has the appearance of a very stumpy-looking refracting instrument, but one instant's examination will show the observer that the usual object-glass is absent at the end of the tube. In fig. 45 we have a Gregorian telescope, mounted on a tripod stand.

Fig. 45.—Gregorian Telescope.

Whilst experimenting on the Gregorian telescope, Newton made certain improvements in its construction, which we shall proceed to describe. A glance at fig. 46 will show that the path of the rays is much more simple than in the instrument we have just noticed.

Fig. 46.—Section of a Newtonian Telescope

The rays of light A B are first reflected from the concave mirror M on to the surface of the small plane mirror m, which is placed at an angle of 45°, and reflects them as far as the point A' B', where they form the image to be magnified by the eye-glass. It is therefore at the side of the instrument, and not at the end, as hitherto, that the observer is placed, and at right angles to the path of the rays. Observers looking at an object through a Newtonian telescope for the first time are generally sufficiently astonished to find that there is really no difficulty after all in seeing round a corner. We shall presently return to the subject of Newtonian telescopes, which were abandoned by astronomers for many years, until they were brought into use again by M. Foucault, a distinguished French philosopher.

Towards the end of the last century Sir William Herschel invented and constructed the reflecting telescope which bears his name. His great object was to avoid the loss of light consequent on the double reflection which took place in all instruments constructed up to that time, and he succeeded at last in making a telescope in which the observer looked directly through the eye-piece at the image formed by the mirror, which was inclined in such a manner that the rays were reflected to the lower edge of the open end of the tube. In using this kind of telescope the observer is placed with his back to the object he wishes to examine, a position that is even more astonishing to those unaccustomed to the use of a Herschellian telescope than the one assumed when employing an instrument of the Newtonian construction. This position has the defect of causing a small portion of the rays proceeding from the object to be intercepted by the head of the observer, but the amount of light lost is so small in comparison to the size of the mirror that in practice it amounts to nothing.

The dimensions of the telescope constructed by Herschel were enormous for that day. It measured 40 feet long, and the mirror was 4 feet in diameter. It was supported by a complicated system of scaffolding, pulleys, and cords, and was capable of magnifying an object 6,000 times. It was by means of this splendid instrument that Sir William Herschel made those wonderful discoveries in astronomy which are inseparably associated with his name. With it he discovered the planet Uranus, many of the double stars, and a large number of nebulæ, which up to that time were unknown. His son, Sir John Herschel, inherits his father's talents as an astronomer, and has enriched science with numberless observations and discoveries of the greatest importance made with this fine instrument. Fig. 47 shows the construction of the Herschellian telescope, and the path of the rays may be easily followed by the student without any help from us.

The vulgar, ever prone to make mountains out of molehills, magnified the power of Sir William Herschel's telescope beyond all bounds. Stories were circulated about his having given a dinner in the interior of the tube to a select party of friends, but as the diameter of the telescope was only a little more than 4 feet, the entertainment, to say the least of it, would have proved somewhat inconvenient to the guests. Another story, which was credited by great numbers of people, was that he had discovered inhabitants in the moon, but that he hesitated to make the matter public for fear he should be prosecuted for spreading atheistical notions. In fact, the tales told of Sir William Herschel's telescope were endless, and caused the astronomer great inconvenience by attracting crowds of idle people to the neighbourhood of Slough, where he vainly endeavoured to carry on his investigations in peace and quietness. It was in vain that these silly assertions were disproved again and again. Having once believed them, people were slow to reject them, and the story of the dinner was told over and over again for many years.

Fig. 47.—The Herschellian Telescope.

The instrument above described is one of those known as front view telescopes, on account of the image of the star being reflected from the surface of the mirror, which was placed obliquely at the bottom of the tube in front of the observer, who examined it by means of the eye-piece without any other reflection taking place, thereby effecting a saving of light, which fully compensated for any loss caused by the mirror being placed askew. The concave mirror made by Herschel alone weighed a ton, to say nothing of the enormous tube and its fittings. Herschel had consequently to invent a special apparatus for holding and moving this gigantic instrument. The moving gear consisted of a mass of beams, pulleys and cords, reminding one more of the rigging of a ship than of a philosophical instrument. The apparatus for moving the telescope appeared so complicated to the casual observer, although in reality it was very simple, that it doubtless contributed in no small degree to the propagation of the fanciful stories we have already spoken of.

The performances of this splendid instrument hardly came up to the expectations of those who saw it in progress. Herschel, it is true, was enabled by its means to use a power of from 3 to 6,000, but he could only use these amplifications on a few objects—the planets, for instance, giving so little light under a high power as to become indistinct and misty. In 1802 Baron von Zach, in his Monthly Astronomical Compendium, went so far as to say that this colossal instrument was not of the slightest utility, that no discovery had ever been made with it, and that it ought to be considered merely as an optical curiosity. Subsequent events, however, proved very conclusively that Baron von Zach was utterly wrong in his statements and prophecies.

The telescope constructed by Herschel, although very wonderful for the day in which it was made, has long since been eclipsed by that belonging to Lord Rosse, and erected by his late father at Birr Castle, near Parsonstown in Ireland. It is superior to Herschel's instrument both in point of size, and workmanship. The late Lord Rosse, not fearing that his dignity would be compromised by such an act, went boldly to work, and learned to polish mirrors like an ordinary workman, the consequence of which was that he could bestow unusual pains upon the finishing of the speculum. His Lordship not only learnt the mere handicraft of speculum polishing, but went deeply into the engineering difficulties of the operation, and succeeded in inventing many improvements for diminishing labour and rendering the form of the surface more perfect. The specula ground and polished under Lord Rosse's method are almost entirely free from what is called spherical aberration,—that is to say, all rays proceeding from a single point of light, such as a star, are collected into a single point instead of being scattered in a round mass. This freedom from spherical aberration is of course necessary to produce perfectly distinct images. In his Life of Newton Sir David Brewster calls it one of the most marvellous combinations of art and science yet seen in the world.

The tube of Lord Rosse's instrument is 55 feet long, and weighs 6-1/2 tons. In form it may be compared to the chimney of a steamboat of enormous size. At one end it terminates in a kind of square box, within which is contained the mirror, whose diameter is 6 feet, and which weighs nearly 4 tons. The weight of the whole apparatus is consequently nearly 10-1/2 tons, or four times as much as Herschel's. It is erected on an oblong mass of masonry, 75 feet in length from north to south, between two solid walls nearly 50 feet high, which serve as supports for the mechanism intended to move this enormous tube in all directions. To the walls are also fixed movable staircases with platforms that can be brought up to the eye-piece with the greatest facility, no matter in what position the telescope may be placed. This noble instrument has penetrated space to a distance perfectly unattempted before its existence, and has resolved numerous nebulæ into masses of stars that until then were supposed to be mere clouds of luminous matter. The exact forms of other nebulæ have also been accurately determined by this telescope, which fully deserves the glowing eulogium passed upon it by the Duke of Argyle in his presidential address at the meeting of the British Association at Glasgow, in 1855. "This instrument," said his Grace, "in extending the range of astronomical science as it has done, has been the means of throwing certain doubts upon the laws that govern the motions of the heavenly bodies, and render it possible that certain of the far-distant nebulæ are regulated in their movements by other laws than those to which the members of our own system are subjected."

The clearness with which this telescope exhibits every object within its range is so great that the most distant nebulæ are seen with as great distinctness as the nearest planet. On directing it towards the moon, which is only distant from us about 240,000 miles, the surface of our satellite may be explored with a facility almost as great as that with which we examine the details of a landscape with an ordinary telescope.

Maedler, a German astronomer, who has measured nearly every mountain and valley on the moon's surface with the greatest exactitude, stated some years before Lord Rosse's telescope was perfected that if a monument as large as one of the Pyramids existed on the surface of the moon it could have been readily distinguished by the instruments then in use. With Lord Rosse's telescope we can see the surface of our satellite so much enlarged that a space 220 feet square could be readily perceived by a good observer. This enormous eye, measuring 6 feet in diameter, would hardly show us a lunar elephant; but it is certain that if a troop of buffaloes, or animals analogous to them, crossed the field of vision, they would undoubtedly be perceptible. Masses of troops marching backwards and forwards would also be plainly visible, and we may assert with something like absolute certainty that there are neither towns nor villages in the moon, nor any buildings as large as St. Paul's of London or the colossal railway stations of that metropolis.

This telescope, as we have said before, is the largest hitherto constructed, and cost its noble constructor more than 25,000l. It must also be recollected that it was not a mere scientific toy belonging to an amateur philosopher, but a real working instrument in the possession of a true man of science, who did work with it that will render his name famous while civilization lasts. The present Lord Rosse seems worthy in every way of his father's great name, and has already enriched astronomical science with numerous valuable observations.

We shall finish this chapter by a description of the Newtonian telescope constructed by M. Léon Foucault. The mirror, instead of being made of speculum metal, which is an alloy of tin and copper, is made of glass from the famous manufactory of St. Gobain. The first rough grinding having been finished, it passed into the workshops of M. Secrétan, the optician to the Paris Observatory, to receive its final polish and finishing touches from the hand of M. Foucault himself, the most careful optical tests being applied to it before the commencement of each operation.

The glass mirror having reached the degree of perfection desired, was then silvered on its concave surface by being plunged into a bath of nitrate of silver, dissolved in water, and mixed with certain proportions of gum galbanum, nitrate of ammonia, and oil of cloves. Half an hour in this bath was sufficient for the deposition of a film of silver of sufficient thickness to bear polishing. When finished, the mirror was found to reflect 92 per cent. of the light incident on its surface, the loss in the case of achromatic object-glasses and metal specula being 20 and 35 per cent. respectively.
Fig. 48.—Foucault's Large Telescope. The substitution of a parabolic glass mirror for the ordinary metal speculum offers the triple advantage of greater lightness, increased distinctness, and more brilliant images. Fig. 48 represents the large silvered glass telescope constructed under M. Foucault's direction for the observatory at Marseilles. It measures 32 inches in diameter, and has a focal length of a little more than 16 feet, and is put in motion by clockwork of a very perfect description, so that when once pointed at a star or planet it follows the object, which would otherwise disappear on account of the rotation of the earth. The path taken by the rays is precisely the same as in Newton's telescope, the eye-piece being placed at the side of the tube, which is provided with a movable platform and staircase for the observer.

The optician to whose talent in his art this fine instrument is due, has recently executed several small telescopes upon the same model, at such a price as to bring them within the reach of amateurs with slender purses. The principal part of these telescopes, one of which is represented in fig. 49, (see next page), is the mirror, which is about 4 inches in diameter, and 24 inches' focal length. The body, which is cylindrical, is made of brass, and revolves on two pivots placed horizontally at about one-third of its length from the bottom. The bearings on which the pivots move consist of two upright standards of metal, which are connected at the bottom, and revolve on a pin in the middle of the plate of the tripod stand. They are made of such a height that the lower portion of the instrument may pass between them, when it is necessary to observe objects in the zenith. By the turn of a screw the whole of the upper portion of the instrument may be dismounted and fixed on a lower standard, so that the observer may work sitting down if necessary. The body of the telescope is provided with a finder. One of the great advantages of this form of instrument is that it can be used for observations on the zenith without giving the observer those unpleasant cricks in the neck so inseparable from the use of ordinary telescopes in a nearly upright condition. The mirror will bear a power of 220 diameters,
Fig. 49.—Foucault's Small Telescope. and shows the mountains of the moon, the phases of Mercury and Venus, Saturn and his ring, Jupiter and his satellites, and a large number of double stars and nebulæ. It is provided with a set of eye-pieces, so that any power almost from 50 to 220 diameters may be used at will. The figure on the opposite page will give the amateur a good idea of the form and size of this instrument.