OK OPTICAL INSTRUMENTS. 429 



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 diminisJied by wholly omit 

 ting 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 

 Avas invented, that the best part of the speculum is sacrificed by the perfora- 

 tion. 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. Cassegrain substituted 

 a convex one, placing it within the focal distance of the large speculum, 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 inter- 

 cepted 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 



