110 The Preparation of Microscope Slides 



ing medium containing glycerin can fail to absorb moisture from the air 

 on humid days and to lose it on dry days, so that it is usually better to 

 finish the slide by applying a ring of varnish around the outside. 



Preparation of Thick Objects. Large objects do not usually make good 

 preparations in gum media because they take a very long time to be- 

 come transparent. It is better, therefore, to mount them in resinous media 

 in the manner about to be described; but, before doing so, it is necessary 

 to explain some of the methods whereby a coverslip may be held in place 

 over a relatively large form. It is obvious that unless support is provided 

 for the coverslip, it is bound to tip to one side or the other and thus make 

 the mount relatively useless. 



A good method of supporting a coverslip over a large object is to cut 

 sheets of celluloid of various thicknesses into little squares spaced equally 

 around the edge before the coverslip is applied. These squares may be 

 made of any desired thickness, to accommodate objects of varying size. 

 This method, however, is not satisfactory for objects much more than 0.5 

 mm thick, because the wide rim left at the edge of the cover causes the 

 mountant to dry out very rapidly. 



An alternative is to provide a little box, known as a "cell," and to 

 cement this to the slide before mounting the object in it. Cells are avail- 

 able in a great variety of materials, and their use is described in some 

 detail in Gray's "Microtomist's Formulary and Guide." For elementary 

 purposes, the best cells that can be used are undoubtedly stamped from 

 sheet tin or pewter and may be obtained in thicknesses varying from 

 Y 2 to about \y> mm. They should be cemented to the slide with some 

 cement that is not soluble in balsam. The best general-purpose adhesive 

 is "gold size"; reference should be made to the source cited above for 

 details of this material and the method of its application. Glass cells are 

 available from biological supply houses. Although they look very attrac- 

 tive, there is no real advantage in their use, since the object is usually 

 studied by transmitted light. 



MOUNTING IN RESINOUS MEDIA 



Resinous media are used for wholemounts not only because they per- 

 mit the mounting of stained objects but more particularly because they 

 make the specimen more transparent. This transparency comes from the 

 increase in the index of refraction of the specimen when it is completely 

 impregnated with the resin. These resins, however, are not miscible with 

 water, so that one is forced first to remove the water (dehydration) and 

 then to replace the dehydrant with some material (clearing agent) with 

 which the resin itself is miscible. Before these operations are conducted, 

 the specimen must be killed and hardened ( fixed ) , and it is customary to 



