40 MATERIALS AND PROCESSES OF SLIDE-MAKING 



made in some medium which will preserve the object and hold the coverslip 

 in place. These mounting media are either water miscible, in which case most 

 objects can be placed in them directly, or resinous, requiring extensive prepa- 

 ration of the object. Each type will be described in its turn. 



Mounting in Gum Media. The simplest preparations are those made in 

 water-miscible mountants, which are of far wider utility than is usually realized 

 because of a complete mental block on the part of most microscopists when 

 faced with any mounting medium which is not a solution of a resin in a 

 hydrocarbon. As a matter of fact, most simple objects, such as the scales of 

 fish and animal hairs, may be mounted more readily in aqueous media than 

 in resinous ones. The actual process of mounting is so simple that it is 

 regarded with distrust by those who have come to believe that only through 

 complexity can good results be produced. With these media one merely takes 

 the object to be mounted, places it in the drop of mountant on the slide, and 

 presses a coverslip on top. This process, moreover, is not confined to relatively 

 hard objects but may be applied to many protozoa and other small inver- 

 tebrates. Small invertebrates do not always make satisfactory permanent 

 mounts by this method, for they ultimately reach a refractive index identical 

 with that of the mountants and thus vanish. A temporary mount of a Para- 

 mecium in one of these media, however, will show the. internal structure 

 better than will the average stained mount, while it will also give a clearer 

 indication of what the living object looked like. The most common objects 

 to be mounted by this method are small arthropods; a description of the 

 preparation of a mite in one of these media is given in the second section 

 of this book. 



Finishing Slides in Gum Media. Slides may be left exactly as they are 

 prepared, but this will give a rather clumsy appearance since some of the 

 mountant will exude from under the coverslip. This exudate may be removed 

 by washing with warm water, but it will be some time after this process 

 before the gum at the edges of the coverslip is dried. Moreover, no mounting 

 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 as Wholemounts. Large objects do not 

 usually make good preparations in gum media because they take a very long 

 time to become transparent. It is better, therefore, to mount them in resinous 

 media in the manner about to be described; but, before doing so, it is neces- 

 sary 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 pro- 



