95 



involves a greater amount of color, that an object- 

 ive showing the proper colors of green and violet 

 and having proper resolving power may safely be 

 accepted, or in the choice of objectives between 

 one showing no color but having no resolving 

 power, the other having color and resolving power? 

 the latter is certainly the preferable one. 



When colors of green and violet are not suffi- 

 ciently pronounced on an object when the mirror 

 is in central position, they will become more 

 apparent when the mirror is swung to an oblique 

 position, using for an object a coarse diatom 

 mounted dry. If the mirror is swung to the left 

 the object will be fringed on the right side by 

 yellow-green and on the left by violet color, or if 

 the mirror is swung to the right the conditions are 

 reversed. 



When the objective is not properly corrected it 

 is said to be chromatically under-corrected or over- 

 corrected. 



Under-correction may be judged by central 

 light, after the object has been focused, by slightly 

 increasing the distance between objective and 

 object, when the latter will show a blue color and 

 by decreasing the distance an orange color. Or, 

 will show over-correction, if on increasing the dis- 

 tance the orange will appear and by decreasing the 

 distance the blue. The appearance will also 

 become more pronounced by using oblique light. 



