78 



tion it will be unnecessary to mark with different 

 figures, as the correction will be the same for all 

 powers. For future examinations on the same slide, 

 this will facilitate work and give the assurance that 

 the best results are thus gained without further trial. 

 Mr. Wenham's general rule for obtaining the best 

 correction on objects in general is as follows : " Select 

 any dark speck or opaque portion of the object and 

 bring the outline in perfect focus ; then lay the finger 

 on the milled head of the fine adjustment and move 

 it briskly backwards and forwards in both directions 

 from the first position. Observe the expansion of the 

 dark outline of the object, both -when within and 

 when without the focus. If the greater expansion or 

 coma is when the object is without the focus or far- 

 thest from the objective, the lenses must be placed 

 farther assunder (or opened). .If the greater coma is 

 when the object is within the focus, or nearest the 

 objective, the lenses must be brought closer together 

 (or closed). When the objective is in proper adjust- 

 ment, the expansion of the outline is the same both 

 within and without the focus." * 



Immersion-Adjustable Objectives. As was 



stated before, immersion contact between the objec- 

 tive and cover-glass is made by either water or homo- 

 geneous fluid. The fluid should be kept in a small 

 bottle or phial, the cork of which is pierced to receive 

 a small pointed stick or match, and this should pro- 

 ject sufficiently so that it will enter the fluid about 



