94 



Chromatic Aberration. Up to this point we 

 have spoken of the correction of chromatic aberra- 

 tion by suitable use of flint glass lenses and for the 

 purpose of not making the subject too complex, 

 have purposely refrained from stating that entire 

 freedom from color is impossible in the ordinary 

 combinations of flint and crown glass. The cor- 

 rection is for only two colors of the spectrum, the 

 red and violet, leaving as a residue the other colors 

 which appear as green and purple. These are 

 called the secondary spectrum and on bright objects 

 can easily be discerned. For all ordinary purposes 

 it is not prejudicial to the performance of an 

 objective. It becomes more pronounced in dry 

 objectives of large aperture and when high power 

 eyepieces are used. In properly corrected low 

 and medium powers of medium aperture and high 

 power immersion objectives it is most noticeable, 

 but certainly not to any extent to be objectionable, 

 except when oblique light is used. 



Great care should be used in judging an object- 

 ive by its chromatic correction and one should not 

 be led to false conclusions by the amount of color 

 which an objective shows. It has been a common 

 experience with the w r riter to have had objectives 

 complained of which were properly corrected and 

 which were excellent in every respect except that 

 they showed the secondary colors, so that it may 

 safely be said, remembering that wide aperture 



