OF OPTIC >. 265 



but converge so as to meet in points about q e r ; where LECT. 

 they would form a larger upright image than a d, if the \^-v-v 

 glass R was 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 R is still retained, to en- 

 large the scope or area of the field. 



To find the magnifying power of this telescope, mul- 

 tiply the focal distance of the great mirror by the dis- 

 tance 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. 



I shall here set down the dimensions of one of Mr. 

 Short's reflecting telescopes, as described in Dr. Smith's 

 Optics. 



The focal distance of the great mirror 9.6 inches, its 

 breadth 2.3 ; the focal distance of the small mirror 1.5, 

 its breadth 0.6 : the breadth of the hole in the great 

 mirror 0.5 ; the distance between the small mirror and 

 the next eye-glass 14.2 ; the distance between the two 

 eye-glasses 2.4 ; the focal distance of the eye-glass next 

 the metal 3.8 : and the focal distance of the eye-glass 

 next the eye 1.1.* 



One great advantage of the reflecting telescope is, 

 mat it will admit of an eye-glass of a much shorter focal 

 distance than a refracting telescope will ; and, conse- 

 quently, it will magnify so much the more : for the rays 



Note 69. One of the largest telescopes yet constructed, was contriv- 

 ed by Dr. Herschel under the patronage of his late Majesty This 

 gigantic intrument is about forty feet in length, and the great metallic 

 reflector weighs more than 2000 pounds. A very powerful instrument, 

 constructed on the same principles, and by the same ingenious astrono- 

 mer is now the property of the London Institution, though unfortunately 

 the want of an observatory has hitherto prevented its useful application. 



