OF OPTICS. 261 



distances equal : and the magnifying power of this LECT. 

 telescope is found the same way as that of the last J^~^, 

 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 ef, since all these three are equal. 



When the rays of light are separated by refraction, why the 

 they become coloured, and if they be united again, they ^^^ 



will be a perfect white. But those rays which pass loured 

 ,ii i , j when seen 



through a convex glass, near its edges are more line- tnrouo -h a. 



qually refracted than those which are nearer the middle telescope, 

 of the glass. And when the rays of any pencil are un- 

 equally refracted by the glass, they do not all meet 

 again in one and the same point, but in separate points ; 

 which makes the image indistinct, and coloured, about 

 its 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 

 edges 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 the 

 -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.* 8 



Note 68. Before we dismiss the subject of refracting telescopes, 

 it may be advisable to notice an ingenious combination of glasses now 

 employed in all the best telescopes, by which the instrument is made to 

 produce a colourless image, and, as such, divested of the prismatic 

 tinge so justly complained of by the early observers. Mr. Dollaud 

 appears to have been the first who succeeded in effecting this desirable 

 object, which he accomplished by employing a compound object-glass, 

 formed of three lenses of different refractive powers. The central 

 being a double concave glass, while the two external glasses may be 

 considered perfect double convex lenses. 



To proportion accurately the densities of the glasses to each other, 

 requires much professional practice and attention. Experiments with 

 the lenses, after they are perfectly finished, can alone be depended on. 

 The essential parts of telescopes being few and cheap, the manufacture 

 of them is frequently attempted by individuals for the purpose of 



