66 SELECTION AND USE 



aberration, or both. It might be caused by a want of finish on 

 the surfaces of the lenses, but this is seldom the case in prac- 

 tice, except where the objective has been exposed to some cor- 

 roding fumes or liquids. Old objectives that have been very 

 excellent in their day, sometimes fail in defining power, from 

 the fact that the surface becomes covered with a greasy deposit, 

 very slight, it is true, but just enough to destroy the efficiency 

 of the glass. Objectives in this condition should be returned 

 to the makers to be cleaned. In one case we found that in a 

 lens which failed to show anything clearly, the difficulty arose 

 from the fact that the cement used for uniting the glasses of 

 the combination had become affected. The objective was by a 

 well-known maker, but was over twenty years old 



Achromatism. When an objective shows much color, it 

 fails to define well except by monochromatic light, such as that 

 obtained by passing sunlight through a cell filled with the blue 

 solution of copper in ammonia. A very slight degree of color 

 is not regarded as objectionable, and indeed it has been found 

 almost impossible to secure the requisite angular aperture and 

 absence of spherical aberration without leaving a little color. 

 Some of the best objectives, therefore, show such objects as 

 the P. angulatum with decided colors, and yet well resolved. 



Aberration of Form. An objective may appear to de- 

 fine an object perfectly, and yet give a very distorted figure oi 

 it, just as a cylindrical mirror gives a perfectly definite, though 

 very distorted, image of objects seen reflected in it. Aberra- 

 tion of form may arise either from over or under correction oi 

 the spherical aberration, or from want of hoinogeniety in the 

 glass used for making the lenses, or from a want of perfection 

 in the workmanship the surfaces of the lenses not being per- 

 fectly spherical. Sometimes this defect is shown very clearly 

 on one side of an objective, while the other side is not affected, 

 'and this fact may give rise to very curious results when the 

 objective is tried on different stands, and with oblique light. 

 Owing to a variation in the point at which the screw threada 

 begin in the different stands, the objective, when fairly screwed 

 up, may have a different position in each, as regards the direction 



