Chromatic Aberration. Up to this point we 

 have spoken of the correction of chromatic aberration 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 correction is for only two colors of 

 the spectrum, red and violet, leaving as a residue 

 the other colors which appear as apple green and 

 purple. These form the so-called secondary spectrum 

 and on bright objects this can easily be discerned. 

 For all ordinary purposes the presence of these colors 

 is not prejudicial to the performance of an objective. 

 The secondary spectrum becomes more pronounced in 

 dry objectives of large aperture and when high power 

 eyepieces are used. Even in properly corrected low and 

 medium powers of medium aperture and high power 

 immersion objectives it is noticeable, but certainly 

 not to any extent to be objectionable, except when 

 oblique light is used, provided of course that the other 

 corrections are properly made. 



Great care should be used in judging an objective 

 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 writer to have objectives complained of which 

 were properly corrected and which were excellent in 

 every respect except that they showed the secondary 



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