OPTICS. 301 



in the focus of the eye-glass i k, and the eye on the 

 opposite side of the same glass ; the image is viewed 

 through the eye-glass in this telescope, in the same 

 manner as through the eye-glass in the former one, 

 only in a contrary position; that is, in the same 

 position with the object. 



The three glasses next the eye have all their 

 focal distances equal ; and the magnifying power 

 of this telescope is found the same way as that of 

 the last above, viz. by dividing the focal distance 

 of the object-glass c d by the focal distance of the 

 eye-glass i k, or g h, or e f, since all these three 

 are equal. 



When the rays of light are separated by re- 

 fraction, they become coloured; and if they be 

 united again, they will be a perfect white. But 

 those rays which pass through a convex glass, near 

 its edges, are more unequally refracted than those 

 nearer the middle of the glass. And when the 

 rays of any pencil are unequally refracted by the 

 glass, they do not all meet again in one and the 

 same point, but in separate points ; which make the 

 object indistinct, and coloured about the edges. 

 The remedy is, to have a plate, with a small round 

 hole in its middle, fixed in the tube at m, parallel 

 to the glasses. For the wandering rays about the 

 middle of the glasses will be stopped by the plate 

 from coming to the eye ; and none admitted but 

 those which come through the middle of the glass, 

 or at least at a good distance from its edges, and 

 pass through a hole in the middle of the plate. 

 But this circumscribes the image, and lessens the 

 field of view, which would be much larger if the 

 plate could be dispensed with. 



Besides the telescopes described before, there is 

 also one called a binocular, or double telescope. 



