12 PROPERTIES OF LENSES. 



the lens a decreasing curvature towards the edges, 

 in fact an ellipsoidal form, the difficulty of this in 

 practice was found so great that it was abandoned, 

 and if the form was such that its performance was 

 perfect for parallel rays, it would be inaccurate for 

 all others. 



It is much easier corrected by altering the curves 

 of the two sides. For instance if the plane side of 

 a plano-convex lens be turned towards a very near 

 object, or the convex side towards a very distant 

 one, the resulting aberration would be about one- 

 fourth of that which would occur if the respective 

 positions of the sides of the lens were reversed. 



A method which has been frequently adopted for 

 photographic objectives deserves mention, namely 

 lessening the aperture, by placing a diaphragm in 

 front of or behind the lens which admits the rays at 

 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 barrell-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 



