6 THE PREPARATION OF MICROSCOPICAL OBJECTS 



the slide about gently ; if the dirt moves with the slide, remove 

 the slide and clean it. If the dirt does not move with either 

 the eyepiece or the slide the fault is almost certainly in the 

 objective, which should be removed and examined ; if dirty, it 

 must be cleaned very carefully with a piece of silk or chamois 

 leather. 



V. THE PREPARATION OF MICROSCOPICAL OBJECTS. 



In mounting microscopical objects be careful that your slides 

 and coverslips are thoroughly clean. Slides should be labelled 

 as soon as they are prepared, and should be kept in a box or 

 cabinet in which they lie flat. 



A. Methods of Mounting. 



There are various media in which objects may be mounted. 

 The method of procedure is much the same with all. Put a 

 small drop of the fluid in the middle of the slide, place the 

 object in the middle of the drop, and arrange it with needles in 

 the position desired. Then place the cover-glass carefully on 

 the top, letting it rest by one edge on the slide and supporting 

 the opposite edge by a needle : withdraw the needle gradually 

 so as to let the cover-glass down slowly, and drive out any air- 

 bubbles there may be in the fluid. If any air-bubbles still 

 remain, leave them alone, as they will probably work out by 

 themselves. Be careful not to use too large a drop of your 

 mounting medium, and above all things be careful not to let 

 any of it get on the top of the cover-glass ; should this happen, 

 the cover-glass must be removed at once and the specimen 

 mounted afresh with a clean one. 



The most important mounting media are the following : 



1. Normal Salt Solution : a 0*75 per cent, solution of com- 

 mon salt in water. This is very useful in the examination of 

 fresh specimens of animal tissues, as, unlike water, it has 

 practically no action on them. It cannot be used, however, 

 for making permanent preparations. 



2. Glycerine can be used either pure or diluted with its own 

 bulk of water. If the preparations are intended to be per- 

 manent, a narrow ring of cement must be painted round the 

 edge of the cover-glass to fix it to the slide. For permanent 



