POLYZONAL LENSES. 161 



image is produced by the rays which pass 

 through the middle of the lens, a piece of card 

 with a hole in the centre being placed in front ; 

 but if, keeping the rest of the apparatus in the 

 same position, I change this card for another 

 piece which will only allow the rays to pass 

 through the edge of the lens, you observe how 

 inferior the image will be. In order to get it 

 distinct I have to bring the screen much nearer 

 the lamp ; and so if I take the card away alto- 

 gether, and allow the light to pass through all 

 parts of the lens, we cannot get a perfect image, 

 because the different parts of the lens are not 

 able to act together. This spherical aberration 

 is, therefore, what we try to avoid by building 

 up compound lenses in the manner here shown 

 (fig. 58). Look at this beautiful apparatus, is it 

 not a most charming piece of workmanship? 

 Buffon first and Fresnel afterwards, built up 

 these kind of lenses, ring within ring, each at 

 its proper adjustment, to compensate for the 

 effects of spherical aberration ; the ring round 

 that centre lens is ground so as to obviate what 

 would otherwise give rise to spherical aberration, 

 and the next ring, being corrected in the same 

 manner, you will perceive, if you look at the 

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