optics. 305 



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

 formed. The like being understood of all those 

 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 passing on 

 from the image through the eye-glass S, and 

 through a small hole e, in the end of the lesser 

 tube tt, they enter the eye f, which sees the image 

 a d (by means of the eye-glass) under the large 

 angle c e d, and magnified in length, under that 

 angle, from c to d. 



In the best reflecting telescopes, the focus of 

 the small mirror is never coincident with the focus 

 m of the great one, where the first image I K is 

 formed but a little beyond it (with respect to the 

 eye), as at n ; the consequence of which is, that 

 the rays of the pencils will not be parallel after 

 reflection from the small mirror, but converge so 

 as to meet in points about q t e, r; where they will 

 form a larger upright image than a d t if the glass 

 R were not in their way : and this image might be 

 viewed by means of a single eye-glass properly 

 placed between the image and the eye ; but then 

 the field of view would be less, and, consequently, 

 not so pleasant ; for which reason the glass II is 

 still retained, to enlarge the scope or area of the field. 



To find the magnifying power of this telescope, 

 multiply the focal distance of the great mirror by 

 the distance of the small mirror from the image 

 next the eye ; and multiply the focal distance 

 of the small mirror by the focal distance of 

 the eye-glass ; then divide the product of the 

 former multiplication by the product of the latter, 

 and the quotient will express the magnifying 

 power. 



vol. i. x 



