EARLY HISTORY OF THE ROYAL SOCIETY 



different rays could be avoided, and a telescope of great 

 perfection constructed, provided a substance could be 

 found which would polish finely, and reflect a large part 

 of the light falling upon it ; and further, that the art of 

 communicating to it a parabolic figure could be attained. 

 It may be mentioned that concave mirrors of polished 

 metal had been in use very early for the magnification of 

 near objects, probably long before Giovanni Rucellai's 

 work on bees by this method in 1524. (" Gli Api, Roma, 



1524-") 



This telescope, made by Newton with his own hands, 

 is nine inches long, two inches in aperture, and is stated 

 by Newton to magnify about thirty-eight times. 



In the form of reflecting telescope suggested by Gregory, 

 the light, after having been reflected from the large concave 

 mirror, is received soon after coming to a focus, upon a 

 small concave mirror which sends it back through a hole 

 in the centre of the large mirror, where the image formed 

 is observed by means of a suitable eye-piece. In Newton's 

 construction the light from the large concave mirror, 

 before coming to a focus, is reflected to one side by a 

 small plane mirror placed at an angle of 45 degrees, and 

 passes through a hole in the side of the tube where the 

 image is viewed. A third form of reflecting telescope was 

 afterwards constructed by Cassegrain, in which Gregory's 

 small concave mirror is replaced by one of convex form ; 

 as this can be placed a little within the focal distance of 



the large mirror, the length of the telescope is shortened 



ii 



