APPENDIX. 489 



microscope very carefully down, until the object is just 

 visible, and then to focus accurately with the fine ad- 

 justment. 



3. Never observe with a high power until the object is 

 covered with a cover-slip. If the object appears in- 

 distinct, this may be due to dirt or condensed vapour, 

 on the cover-slip, the objective, or the eye-piece. If the 

 first is in fault the distinctness will vary as the slide is 

 moved. The presence of any foreign particles on the 

 eye-piece can be easily detected by turning it round. A 

 general indistinctness, which is not affected either by 

 moving the slide, or by rotating the eye-piece, must be 

 due to the objective having become dirty, or injured. 



4. Should the objective be dirty, it must be cleaned with a 

 soft linen rag, or with a piece of wash-leather, never 

 with a coarse cloth. The greatest care must be taken 

 that the objective does not become dirty, for even the 

 most careful cleaning is likely to injure the lens. In 

 order to keep the objective clean, it is essential that no 

 more fluid should be used in mounting, whether tem- 

 porarily or permanently, than will exactly fill the space 

 under the cover-slip. 



5. During observation the focussing should constantly be 

 slightly varied, by means of the fine adjustment, as this 

 greatly aids in getting a clear idea of the object. 



6. It is best to cultivate a habit of using the microscope in 

 the vertical position, so that the stage is horizontal. An 

 inclined position of the instrument is only admissible in 

 examining permanent preparations. 



7. The body of the microscope should always fit accurately, 

 but not tightly, in its tube. It should be oiled if it does 

 not work up and down quite smoothly. 



8. A micrometer of some kind is indispensable, and the 

 purpose to be aimed at in its use is the knowledge of the 

 size of objects under examination. An eye-piece mi- 



