196 Experimental Philosophy. [Lecture 13, 



a succession of images, in the order of the pris- 

 matic colours from Rr to Vv. As in the former 

 case, these different images will form but one to 

 the eye of the spectator ; but it will be imperfect 

 and coloured at the edges, as well as the field of 

 view. Various remedies were devised for this 

 defect. At length Mr. Dollond, finding that flint 

 and crown glass had different refracting powers, 

 and that crown glass (the common window glass) 

 dispersed the rays of light less than any other, 

 adapted two convex glasses of crown glass to a 

 double concave of flint glass (which has the great- 

 est dispersive power), so as exactly to fit, and by 

 that means made them counteract each other, so 

 that the field of view is presented perfectly colour- 

 less. These telescopes, therefore, are called achro- 

 matic (or colourless) telescopes. 



The reflecting telescope accomplishes- by re- 

 flecting the rays issuing from any object, what 

 the last did by refracting them. Let ab, (PL 

 XVI. fig. 76) be a distant object to be viewed ; 

 parallel rays issuing from it, as ac and bd, will be 

 reflected by the metallic concave mirror, cd to st y 

 and there brought to a focus, with the image a 

 little further and inverted, agreeably to the effect 

 of a concave mirror on light, as formerly described. 

 The hole in the mirror cd does not distort or hurt 

 the image st 9 it only loses a little light ; nor do 

 the rays stop at the image st ; they go on, and 

 cross a little before they reach the small concave 

 mirror en : from this mirror the rays are reflected 

 nearly parallel through the hole O, in the large 



