12 PROPERTIES OF LENSES. 



the centre, but cuts off those at the edges, it will be 

 evident that this lessens the chromatic aberration at 

 the same time. 



The diaphragm has other advantages, it enables 

 objects lying at different distances from the lens to 

 appear distinct, in other words, the penetrating 

 power is increased. Its disadvantages are that it 

 produces distortion, if placed in front of a lens the 

 image of a square becomes barrel-shaped, if behind 

 the reverse. It lessens the light, and diminishes the 

 Angular Aperture, upon the extent of which the 

 most important qualities of an objective depend. 



The aberration of a concave lens being exactly 

 opposite to that of a convex lens, one may be made 



to correct the other without decreasing the magnify- 

 ing power to any great extent. The lens A Fig. 10 

 completely destroys the spherical aberration of the 

 lens B, for it does not change the position of the 

 rays situated near its centre as much as it does those 

 at the edges, owing to its increasing prismatic form. 

 If advantage be taken at the same time of the 

 different relations between the refractive and disper- 

 sive powers of different kinds of glass, and if the 

 lenses which correct the spherical aberration by their 



