352 APPENDIX. 



With a higli power, lower the tube till it is -| of 

 an inch from the object, and then slowly twist 

 the tube down, moving the slide about, till the 

 object just becomes visible, then focus by means of 

 the fine adjustment. 



Direct sunlight should not be used to illuminate the 

 field ; in selecting a permanent position to work with 

 the microscope, it is best to face the north. 



With high powers, use the corresponding small dia- 

 phragm; otherwise although the field may look 

 brighter, the outlines of the cells, etc. will not be so 

 well defined. 



The student should accustom himself to keep both eyes 

 open when using the microscope, and to use either the 

 right or the left eye for looking at specimens. The 

 fatigue of microscope work is thus very much lessened. 

 It generally requires a little practice to keep both eyes 

 open, but a few minutes' trial every time that the 

 microscope is used will soon overcome the difficulty. 



When an object appears dim, it may be caused by some 

 fault in the specimen itself, or by the object-glass 

 not being clean. In the latter case the dimness 

 remains, whether the object is in focus or not. Dust 

 upon the lenses of the eye-piece can be recognized 

 by the outlines of the particles appearing well de- 

 fined, not dim and diffuse. 



Zeiss' camera Incida. 



The ring (r) Fig. 7, is slipped over the tube of the micro- 

 scope (t), and the ocular (o) inserted ; the object to be 

 drawn is focussed, and the prisms are brought over 

 the ocular in the position shewn in Fig. 7. (The 

 upper part of the figure represents a side view ; the 



