538 FOOT AND STROBELL. [Vol. XVII. 



The simplest method of determining the proper focusing 

 lens is to photograph a minute cytological detail, using a focus- 

 ing lens of medium strength, for example, — 5.D. If your 

 negative shows too high a focus {i.e., too high a plane), you 

 need a weaker lens ; if too low a focus, a stronger lens, and 

 successively stronger or weaker lenses can be placed on the 

 eye-piece, until one shows the exact focus reproduced by the 

 negative, that being the lens you should have used. A readjust- 

 ment of the focus with the right lens will give an exact repro- 

 duction of the finest details. With this method it is necessary 

 to take only one photograph in order to determine the proper 

 focusing lens, instead of taking several, as suggested in an 

 earlier paper. 



The spherical lenses are inexpensive and a series from 

 — i.D. to— 10. D., will be found useful. 



