ON OPTICAL INSTRUMENTS. 335 



astronomical telescope ; a concave speculum or mirror being substituted 

 for the object glass, and the eye glass being so placed as to magnify the 

 image formed by the speculum. But since the speculum, if it received the 

 principal rays perpendicularly, would send them back in the same direc- 

 tion, it is necessary, in this construction, to have them reflected somewhat 

 obliquely, the speculum being a little inclined to the axis of the telescope, 

 in order that the light may have free access to it. An arrangement of this 

 kind was proposed long ago by Maire,* but it has been very little employed 

 before Dr. Herschel's time. This excellent philosopher and mechanic has 

 carried the perfection of his telescopes to a degree far exceeding all that 

 could have been expected from the labours of former opticians. His in- 

 struments allow him to extend the linear dimensions of his objects several 

 thousand times : but he commonly finds it more eligible to employ only 

 powers of 5 or 600, which afford a much stronger illumination. (Plate 

 XXVIII. Fig. 406.) 



The Newtonian reflector has a plane speculum placed in its axis, at the 

 inclination of half a right angle, which intercepts the rays about to form 

 the image, and throws them into the focus of an eye glass fixed in the side 

 of the tube. The plane speculum which he employed was the posterior 

 surface of a rectangular prism of glass, which produces a total reflection : 

 but Dr. Herschel has found that the sources of error are diminished by 

 wholly omitting this speculum. (Plate XXVIII. Fig. 407.) 



In the Gregorian telescope, the object speculum is perforated, and the 

 image formed by it is received into the focus of a smaller concave speculum, 

 which returns it to be viewed through the aperture by the eye glasses. It 

 has been objected to this form of the reflecting telescope, which is the first 

 that was invented, that the best part of the speculum is sacrificed by the 

 perforation. But Dr. Herschel has found that the image formed by the 

 external part of a speculum is in general more perfect than that which is 

 formed by the central part. (Plate XXVIII. Fig. 408.) 



For the smaller concave speculum of Gregory, Mr. Cassegraint substi- 

 tuted a convex one, placing it within the focal distance of the large specu- 

 lum, so as to form the first actual image nearly in the same place as the 

 second image of the Gregorian telescope ; but this image is inverted. The 

 instrument has some advantage in theory, with respect to the perfection of 

 the focus ; but it is little used. (Plate XXVIII. Fig. 409.) 



Dr. Smith's reflecting microscope resembles Cassegrain's telescope, but 

 the rays of light are first admitted through a perforation in the small 

 speculum, that part of them which tends to fall immediately on the eye 

 being intercepted by a screen. The convexity of the one mirror is nearly 

 equal to the concavity of the other ; and the instrument, although seldom 

 employed, is said to succeed extremely well. (Plate XXVIII. Fig. 410.) 



The image of a very distant object, formed by a speculum of any kind, 

 is considerably less curved than that which is depicted by a lens of equal 



* Mach. Approuv. vi. 61. Herschel on his Forty-foot Telescope, Ph. Tr. 1795, 

 p. 347. See also Herschel, ibid. 1782, p. 173; 1786, p. 499; 1800, p. 49; 1803, 

 p. 214. 



t Journal des Savans, 1672. See Newton, Ph. Tr. 1672, p. 4056. 



