OF OPTICAL INSTRUMENTS. 



79 



452. Definition. Dr. Herschel's re- principal rays are received and reflected 

 fleeting telescopes resemble, in their effects, somewhat obliquely, in order to allow the 

 the simple astronomical telescope: but the light free access to the speculum. 



[ 



453. Definition. The Newtonian re- for the convenience of fixing the eyeglass in 

 flector has a plane speculum placed in its the jde of the tube. 



axis, at the inclination of half a right angle. Scholium. Dr. Herschel's construction differs from 



this only in the omission of the plane speculum. 



r 



454. Definition. In the Gregorian te- smaller concave speculum, which also re- 



lescope, the object speculum is perforated, verts it : it is afterwards submitted to one 



and the image formed by it, is transmitted or more eyeglasses, 



through the aperture, after reflection from a ~ " 



4.55. Definition. The telescope of Cas- the first image falls near its principal focus, 



segrain has a convex speculum instead of and the second is thrown back into the focus 



Gregory's smaller concave, placed within the of the eyeglass. 



focal distance of the large speculum, so that Scholium. The image is here inverted. 



456. Definition. Dr. Smith's reflect- and prevented by a screen from falling im- 



ing microscope resembles Cassegrain's tele- mediately on the eye. The radii of the sur- 



scope, but the rays of light are first admitted faces ar6 equal, 

 through a perforation in the small speculum. 



