APPARATUS AND METHODS. 15 



the clean cover flat, but prop it against some clean and 

 dry object until it is required for mounting. Always clean 

 and dry the slides and covers when done with ; do not put 

 them away wet or dirty. 



In mounting, place on the centre of the slide a drop of 

 the mounting fluid. Place the specimen in this, and care- 

 fully lower the cover-glass, in such a way that no air 

 bubbles shall be included in the preparation. To do this, 

 hold the clean dry cover at one side of the drop of fluid, 

 in a tilted position, then place a mounted needle under the 

 cover at the other side and gently lower the cover by 

 withdrawing the needle. If this is done carefully, very 

 little fluid will flow from below the edges of the cover, 

 and no air bubbles should be included. 



If water flows over the upper side of the cover, or if 

 air bubbles get in, remove and dry the cover and try 

 again. With a little practice one can accurately judge 

 the size of the drop of fluid according to the size of 

 the cover-glass and the thickness of the specimen. 

 Any slight excess of fluid on the slide around the edges 

 of the cover can be soaked up with torn bits of blotting- 

 paper. 



It is of the utmost importance that no water or other 

 mounting fluid should reach the upper side of the cover or 

 the surface of the objective ; if either of these get wetted, 

 or if liquid gets on the lower side of the slide or on the 

 stage, draw up the tube of the microscope, and thoroughly 

 clean and dry the objective, the slide, and the stage, also 

 removing the cover-glass and making a new preparation. 

 Absolute cleanliness and care in the use of mounting 

 fluids and reagents must be observed, otherwise much 

 time may be wasted. A cover-glass must invariably be used 

 with the high-power objective. 



Some other points worth noting *are the following. 

 Never use more than one cover on a slide, and place this 

 as nearly as possible at the middle of the slide. Never 

 press upon the cover, unless there is some definite object 

 in doing so ; if a section is too thick, pressure will only 

 make it worse, and if the section is thin it will simply be 

 ruined by pressure. A section must not be allowed to get 



