160 Tlie Preparation of Microscope Slides 



coverslip of suitable size (Fig. 126) is held at an inclined angle with a 

 bent needle and slowly lowered so as to exclude all air bubbles. Then 

 the edges of the slide are wiped roughly, and the slide is returned to the 

 hot table shown in Fig. 125 to evaporate the solvent used for the resin. 



Although this is the conventional method of operation, it is by no 

 means the best. In particular, there is a tendency to have a higher con- 

 centration of solvent along the edges of the coverslip than in the center, 

 and it also takes a surprisingly long space of time for all the solvent to 



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Fig. 126. Placing the coverslip on serial-section slide. 



be removed. It is much better, if one can spare the time, to place a rela- 

 tively thin coat of mounting medium on top of the slide and then to 

 allow the solvent to evaporate from this on the surface of a hot plate. 

 There is no risk that the slide will dry out, because the mountant will act 

 as a varnish. It is needless to say that in fine work it is necessary to cover 

 the slide and hot plate with some dustproof cover while this is going on. 

 The next day the slide is examined and, if it appears to be sufficiently 

 varnished, the coverslip is placed on the surface. Then the whole slide 

 is warmed, while maintaining steady pressure, above the softening point 

 of the resin. The slide will be hardened as soon as it is cooled and may 

 be cleaned and put away. This custom of evaporating the solvents from 

 the surface of the slide rather than from the edge of the coverslip is con- 

 sidered old-fashioned nowadays, but there is no doubt that it produces a 

 better and more durable slide than does the more usual procedure. 

 It must not be imagined that, just because all these directions have 



