144 



Spherical aberration is corrected when the bands 

 or lines show distinctly without any nebulous 

 fringe, and thus indicate the proper thickness of 

 cover to use. 



Chromatic correction may be judged by the 

 character of color bands which show with oblique 

 light. If the bands are narrow and the compli- 

 mentary colors of the secondary spectrum, yellow- 

 green to apple-green on one side and violet on the 

 other, indicates the best correction. For deter- 

 mining the thickness of cover glass and best cor- 

 rection for spherical aberration we refer to the 

 cover glass gauge designed by the writer. 



If the cover glass is first measured with this 

 before the object is mounted, the tube length is 

 given for proper correction for each thickness and 

 can be read off on the drum. The scale provides 

 for J, \, i, and, -fa inch objectives of the long tube 

 length and for the ^ inch of the short tube length. 

 It also gives the thickness of cover in millimeters 

 and inches, and thus indicates the true position of 

 adjustment-collar in adjustable objectives. 



Dry, Adjustable Objectives. Adjustable ob- 

 jectives, or objectives with collar correction are 

 those in which there is a provision to vary the dis- 

 tance between lenses, in order to make proper cor- 

 rection for spherical aberration with facility. They 

 involve a high degree of mechanical perfection and 

 are therefore more expensive than objectives with 



