103 



TELKSCOPE, HISTORY OF THE. 



TELESCOPK, HISTORY OF THE. 



no 



the field, they were qpt esteemed. Pere de Rheita, about the same 

 time, constructed for telescopes eye-tubes containing three lenses, 

 which, he observes, afford a better image than those with two. The 

 same person was the inventor of what is called a binocular telescope, 

 that is, an instrument which consists of two telescopes having equal 

 magnifying powers, and placed near each other in such positions that 

 an object might be observed with both eyes at the same time. 

 Attempts have been since made to revive this invention ; but the 

 advantages, if any there be, are more than compensated by the trouble 

 of directing the two tubes to the object. By combining, however, the 

 stereoscopic principle, some advantages are likely to arise, as in the 

 case of the binocular microscope. 



The magnifying power of a dioptrical telescope increasing with the 

 ratio which the focal length of the object-glass bears to that of the eye- 

 and since, by increasing the focal length of the former without 

 increasing its diameter, the coloured border round the image is dimi- 

 nished so that vision is rendered more distinct, the opticians of the 

 17th century were induced to form, for object-glasses, lenses which 

 were segments of very great spheres that is, lenses of great focal 

 lengths. Campani, at Bologna, by order of Louis XIV., made tele- 

 scopes having object-glasses whose focal lengths were as great as 136 

 feet; and with such, Cassini, in 1671, discovered the satellites of 

 Saturn. 



Huyghens, who was an ingenious mechanic, as well as a good 

 philosopher, contrived to use an object-glass of long focus for astrono- 

 mical purposes without placing the system of lenses in a tube. On 

 the top of a long pole which was planted vertically in the ground, he 

 mounted the object-glass, having fixed it in a frame with joints so that 

 its axis could be moved in any direction by means of a string which 

 was held in the hand of the observer ; and the axis being in a line 

 passing through the celestial body, a short tube containing the eye- 

 glasses was fixed to a stand near the ground with its axis in the same 

 direction. An ae'rial rtfracti/r (as this kind of instrument was called), 

 having an object-glass 123 feet focal length, was made by Huyghens 

 and presented to the Royal Society ; and with it Dr. Bradley made 

 some of his astronomical observations. Pound used it to furnish the 

 diameters of Jupiter and Saturn, and the elongations of their satellites, 

 calculated on by Newton in his ' Principia.' It is described by Huy- 

 in his ' Astroscopia Compendiaria,' which waa printed at the 



in 1 684. M. Anzmit is said to have executed an aerial refractor 

 at 1'aria of 600 feet focal length ; but this proved unmanageable. But 

 the chief merit of Huyghens as an improver nf astronomical telescopes 

 consists in his construction of an eye-piece with two lenses so combined 

 as both to enlarge the field of view and diminish the alienations 

 produced by their spherical forms. 



There is some probability that the elder Digges had contrived an 

 instrument which constituted a species of catoptric or reflecting tele- 

 scope ; but, on account of the obscure manner in which the instrument 

 is described, it will be scarcely necessary to notice further his claim to 

 the honour of the invention. It appears that Pere Mersenne, in his 

 correspondence with Descartes, and in his 'Catoptrics' (1651 (.suggested 

 the idea of a concave spherical mirror to be used, like the principal 

 lens of a dioptric telescope, for forming in its focus an image of an 

 object ; and that this image being viewed through a convex eye-glass 

 >if pro|*r curvature, the original object would appear to be magnified. 

 Descartes, in his reply to Mersenne, which is said to have been written 

 in 1 >'.:'.!, makes several objections to the scheme, and no effort was then 

 made to put it in practice. But the great length of the dioptric tele- 

 scopes which were then in use rendering the management of them very 

 inconvenient, ingenious men were induced to attempt a construction 

 in which, with equal magnifying power, much smaller dimensions 

 might In- pin|i]oyeil. Mr. .lames Oregory of Edinburgh, in his ' Optica 

 'ul.lished a suggestion for forming a telescope by 

 mean* of the image at the focus of a concave speculum. The mirror 

 was to be of polished metal with a paraboloidal surface, which by the 

 ities of that curve would cause all rays incident upon it in 

 directions parallel to the axis to converge accurately at one |">>nt. 

 It is uncertain whether Oregory had any knowledge of Mersenne's 



, or whether the idea originated with himself; but thin is of 

 little consequence, for not being able to find an artist who could exe- 

 cute such a speculum, though he came to London for the purpose, the 

 suggestion was abandoned, and men of science continued to direct 



inquiries to the means of improving dioptric telescopes. 



When, however, Newton had discovered the unequal refrangiliility 

 of light, and had ascertained that the aberration produced by this 

 cause about the focus of a lens was many hundred times greater 

 than that which was caused by the spherical form of the glass, he 

 gave up the hope of being able to construct refracting telescopes 

 which should be free from this defect, and applied himself to 

 the formation of specula for those of the catoptric kind : the image 

 formed by reflection from a mirror being free from what is called the 

 rhromatic aberratinn, and consequently incomparably more dictim-t 

 than one which is formed by the refraction of light in a lens of any 

 transparent medium. 



In the beginning of 1669, Newton having obtained a compos!* 

 metals whiea appeared likely to serve for a mirror, began with hi.s own 

 hands to grind its surface to a spherical form ; and early in the year 

 1672 he completed two telescopes : of the construction and performance 



of these instruments he sent to the Royal Society an account which 

 was read in the January of that year. The radius of the concave metal 

 in one of them was 13 inches, and the telescope magnified about 38 

 times. The rays, before forming an image in the focus of the speculum, 

 were intercepted by a glass prism, or a plane mirror, and the image 

 formed after this second reflection was viewed by a convex eye-glass 

 which was fixed for the purpose in the side of the tube. In the 

 telescope proposed by Gregory, the rays in each pencil of light, after 

 crossing at the focus of the great speculum, were to fall upon the 

 surface of a small ;concave mirror ; and by this being again reflected, 

 they were to form a second image near the anterior surface of the first 

 speculum : through a perforation in the latter the image was to be 

 viewed ; a convex lens being interposed between the image and the eye 

 of the observer. This has been always called the Gregorian telescope ; 

 and in 1672, the year in which Newton completed his reflecting 

 telescopes, M. Cassegrain, in France, proposed one which differed from 

 that of Gregory only in the rays reflected from the great speculum 

 being intercepted by a small convex mirror ; from this the rays of each 

 pencil were again reflected, and they were made to form an image near 

 the anterior surface of the great speculum; this image was to be 

 viewed through a convex lens behind an aperture in the latter specu- 

 lum, as in the telescope of Gregory. It does not appear that M. 

 Cassegrain constructed such a telescope, but it may be observed that 

 the image formed- after reflection from the convex speculum would be 

 more free from the aberration caused by the surfaces of the mirrors, 

 and would also be rather greater, than that which is obtained from 

 the concave speculum of Gregory, or the plane one which was used by 

 Newton. 



The first reflecting telescope, in which the great speculum was 

 perforated so that objects could be viewed by looking directly at them, 

 was executed' by Dr. Hooke, and produced before the Royal Society in 

 February, 1674. But the difficulty of obtaining metal proper for the 

 purpose, and of giving it a perfectly spherical form, for a long time 

 prevented reflecting telescopes from attaining the desired degree of 

 perfection. In 1718 Mr. Hadley succeeded in executing two teles- 

 copes, each about five feet long, which were considered good ; and he 

 gave, in the ' Philosophical Transactions' (1723), a description of the 

 methods employed in their construction. By his advice Dr. Bradley, 

 who was then professor of astronomy at Oxford, in conjunction with 

 Mr. Molyneux at Kew, applied themselves to the construction of these 

 instruments : having executed one which was satisfactory, they in 

 1738 instructed Scarlet and Hearne, two London opticians, in the 

 processes which they used, and these artists presently succeeded in 

 making good reflecting telescopes for general sale. Mr. James Short 

 of Edinburgh, also soon afterwards distinguished himself by his skill 

 in forming such telescopes : he attempted at first to make the principal 

 speculum of glass, but finding that this material had not sufficient 

 steadiness to preserve the form of its surface, he devoted himself to 

 the improvement of metallic specula, and succeeded in giving them, 

 it ia supposed, a correct parabolic figure, by which means hi.s 

 telescopes admitted of larger apertures than any that had before been 

 made. 



The processes adopted by Mr. Mudge in grinding and polishing the 

 mirrors for reflecting telescopes, and in giving them the parabolic 

 figure, may be seen in the ' Philosophical Transactions ' for 1777. See 

 also SPECULUM. 



But the reflecting telescope was carried to a high degree of excellence 

 by Dr. (afterwards Sir William) Herschel. This distinguished astro- 

 nomer, while residing at Bath, employed his leisure hours in grinding 

 and polishing specula, with which he formed telescopes, both of the 

 Newtonian and Gregorian kinds; and about the end of 1788, that is 

 subsequently to the discovery of the planet which is sometimes called 

 by his name, being aided by the liberality of the king (George III.), he 

 began the formation of a speculum four feet in diameter and forty feet 

 in focal length : the telescope to which it appertains is of the Newtonian 

 kind, the observer being placed in a seat near the open end of the tube, 

 and viewing the image through a system of eye-glasses. With this 

 telescope, which was completed in 1789, objects are magnified about 

 6500 times ; and on the night after it was finished, Dr. Herschel dis- 

 covered the sixth satellite of Saturn. The attempts that have been 

 made to form a reflecting telescope possessing a higher degree of 

 perfection than that of Herachel will be noticed presently. 



While the improvement of reflecting telescopes was in progress, the 

 efforts to combine glass lenses in order to diminish the coloured fringes 

 by which the images in dioptrical telescopes are surrounded were not 

 entirely neglected; and as early as 1729, a private gentleman, Mr. 

 Chester More Hall, of Essex, influenced, it appears, by an opinion that 

 the humours of the eye are combined so as to correct the dispersions 

 which each alone would produce in the different kinds of light, con- 

 trived to combine two lenses of different kinds of glass in such a way 

 as to form an image which was free from colours : it is added that 

 telescopes with such object-glasses were in the possession of several 

 individuals many years afterwards. (' Gent. Mag.', October, 1790 ; 

 ' Phil. Mag.', November, 1798.) 



In 1747, Euler, guided also by the constitution of the eye, conceived 

 the possibility of forming a lens compounded of two hollow spherical 

 segments of glass, inclosing water between their concave sides, which 

 should be free from the chromatical and spherical aberrations ; and in 



