[ "250 ] [Book III. 



CHAP. VIII. 



OF TELESCOPES AND OTHER OPTICAL 

 INSTRUMENTS. 



Principles on which tbefg Inftrumenis are nnJiruied % -Dcft:?li. Telt- 

 fcope of Galileo. Micro/cope. RefleQing Telescope. Camera O- 

 Jcura.Magu Lantbcrn. 



LE T QJP (Plate XXI. fig. 43.) reprefent a very 

 diftarit object, and let the rays coming from it, 

 before they fall upon the eye, be intercepted by two 

 convex lenfes, placed at a diflance from each other 

 equal to the fum of the focal lengths. The lens A B 

 is called the object-glafs, from its being oppofed to the 

 object ; C D the eye-glafs, from its being neareft to 

 the eye. Since the object is at a very great diftance, 

 the image made by refraction, q p, will be made at a 

 diflance from the object-glafs equal to its focal length, 

 and confeqnently the image is' diftant from the other 

 glafs exactly its focal length, and the rays diverging 

 from any point of the image will, after refraction by 

 the eye-glafs, move parallel to the line drawn from 

 that point through the center of the glafs. The pro- 

 grefs of the rays then, by which the object is feen, is 

 cafily traced. A ray P will be refracted by, the ob- 

 ject-glafs in the direction P A^D, and by the eye- 

 glafs in the direction D O parallel to p E. The angle, 

 therefore, urHer which the image is feen, is equal to 

 qF.p, ,and the angle by which the object would have 

 been feen by the naked eye is equal to qFp; con- 

 feqnently the magnitude of the object feen by the 

 :! eye is to its magnitude, feen through the glaftes, 



as 



