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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, it' any there be, are more than compensated 

 by the trouble of directing the two tubes to the object. 



The magnifying powerof adioptrical telescope increasing 

 with the ratio which the focal length of the object-glass 

 bears to that of the eye-glass, 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 op- 

 ticians of the seventeenth century were induced to form, 

 for object-glasses, lenses which were segments of very 

 great spheres, that is, lenses of great focal lengths. Cam- 

 pani at Bologna, by order of Louis XIV., made telescopes 

 having object-glasses whose focal lengths were as great 

 as 136 feet, and with such, Cassini, in 1671, discovered 

 the satellites of Saturn. [CAUPANI.] M. Auzout is said 

 to have made a lens of 600 feet focal length, but it does 

 not appear that he was able to use it as a telescope. 



Huygens, who was an ingenious mechanic as well as a 

 good philosopher, contrived to use an object-glass of long 

 focus for astronomical purposes without placing the sys- 

 tem of lenses in a tube. On the top of a long pole which 

 \\:is 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 aline passing through the celestial body, 

 a short tube containing the eye-glasses w as fixed to a stand 

 near the ground with its axis in the same direction. An 

 instrument of this kind having an object-glass of 123 feet 

 local length, was made by Huygens and presented to the 

 Koyal Society ; and with it Dr. Bradley made some of his 

 astronomical observations. It is described by Huygens in 

 his ' Astroscopia Compendiaria,' which was printed at the 

 Hague in 1684. But the chief merit of Huygens as an 

 improver of astronomical telescopes consists m his con- 

 struction of an eye-piece with two lenses so combined as 

 both to enlarge the field of view and diminish the aberra- 

 tion* produced by their spherical forms. 



There is some probability that the elder Digges had 

 contrived an instrument which constituted a specie* of 

 catoptric, or reflecting, telescope; 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 Pi-re Mersenne, 

 in his correspondence with Descartes, and in his 'Catop- 

 trics' (1651j, 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- 

 of proper curvature, the original object would ap- 

 pear to be magnified. Descartes, in his reply to Mer- 

 senne, which is said to have been written in 1639, makes 

 ;il objections to the scheme, and no effort was then 

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

 dioptric telescopes 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 

 be employed. Mr. James Gregory of Edinburgh, in his 

 'Optica Promota' (1663) published a suggestion for form- 

 ing a telescope by means 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 proper- 

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

 directions parallel to the axis to converge accurately at 

 one point. It is uncertain whether Gregory had any 

 knowledge of Mereenne's treatise, or whether the idea 

 originated with himself; but this is of little consequence, 

 for not being able to find an artist who could execute 

 such a speculum, though he came to London for the pur- 

 pose, the suggestion was abandoned, and men of science 

 continued to direct their inquiries to the means of im- 

 proving dioptric telescopes. 



When, however, Newton had discovered the unequal 

 refrangibility of light, and had ascertained that the aber- 

 ration produced by this cause about the focus of a lens 

 many hundred times greater than that which was 

 canned 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 him- 



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 chromatic aberration, and 

 consequently incomparably more distinct than one which 

 is formed by the refraction of light in a lens of any trans- 

 parent medium. 



In the beginning of 1669, Newton having obtained a 

 composition of metals which appeared likely to serve for 

 a mirror, began with his own' hands, to grind its surface 

 to a spherical form ; and early in the year 1672 he com- 

 pleted two telescopes : of the construction and per- 

 formance of these instruments he sent to the Royal So- 

 ciety 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 re- 

 flection was viewed by a convex eye-glass which was 

 fixed for the purpose in the side ol 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 specu- 

 lum : 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. Cas- 

 segrain, in France, proposed one which differed from that 

 of Gregory only in the rays reflected from the great spe- 

 culum 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 specu- 

 lum 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 telescopes, each about five feet long, which 

 were considered good ; and he gave, in the ' Philosophical 

 Transactions' (1723), a description of the methods em- 

 ployed in their construction. By his advice Dr. Bradley, 

 who was then professor of astronomy at Oxford, in con- 

 junction 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 

 ood reflecting telescopes for general sale. Mr. James 

 ihort, of Edinburgh, also soon afterwards distinguished 

 himself by his skill in forming such telescopes : he at- 

 tempted at first to make the pnncipal 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 is supposed, a correct parabolic figure, by 

 which means his 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 ' Phi- 

 losophical Transactions' for 1777. See also SPECULUM 

 MKTAL. 



But the reflecting telescope was destined to receive the 

 liighest power of which perhaps it is susceptible from the 

 hands of Dr. (afterwards Sir William) Herschel : this dis- 

 tinguished astronomer, while residing at Bath, employed 

 his leisure hours in grinding and polisliing specula, with 



