Chap. 8.] Reflecting Telefcope. 257 



mity A of the object AB ; but by paffmg through the 

 convex glafs R in their way, they meet and crofs 

 fooner, as at a> where that point of the erect image is 

 formed. The like being underflood of all thofe rays 

 which flow from the intermediate points of the object 

 between A and B, and enter the tube T T, all the 

 intermediate points of the image between a and b will 

 be formed ; and the rays pafiing on from the image 

 through the eye- glafs S, and through a fmall hole 

 t in the end of the leffer tube //, they enter the eye/j 

 which fee* the image a b (by means of the eye-glafs) 

 under the large angle c e d> and magnified in length 

 under that angle from c to d. 



In the bed reflecting telefcopes, the focus of the 

 fmall mirror is never coincident with the focus m of 

 the great one, where the firft image I K is formed, 

 but a little beyond it (with refpect to the eye) as at 

 n ; the confequence of which is, that the rays of the 

 pencils will not be parallel after reflexion from the 

 fmall mirror, but converge fo as to meet in points 

 about q> e, r, where they would form a larger upright 

 image than a b, if the glafs R was not in their way, 

 and this image might be viewed by means of a fingle 

 eye-glafs properly placed between the image and the 

 eye ; but then the field of view would be lefs, and 

 confequently not fo pleafant ; for that reafon the glafs 

 R is itill retained to enlarge the fcope or area of the 

 field. 



To find the magnifying power of this telefoope, 

 multiply the focal diftance of the great mirror by the 

 diftance of the fmall mirror from the image next the 

 eye, and multiply the focal diftance of the fmall mirror 

 by the focal diftance of jche eye-glafs j then divide the 

 product of the former multiplication by that of the 



VOL. I, S latter, 



