14 



THE ART OF MAKING MICROSCOPE SLIDES 



Cements 



given in Chapter 28 under V 13.1. This is 

 an excellent dead-black cement but, since 

 it has a gold-size base, is very slow drjdng. 

 The best colored backgrounds are the old 

 wax and resin formulas, particularly those 

 of Martin 1872, Oschatz 1842, and Mende- 

 leef (1942); the formulas for these are in 

 Chapter 28 under the heading V 12.2. 

 These media have the advantage that 

 they are thermoplastic so that they may 

 be used both to provide a smooth back- 

 ground and to secure the adhesion of the 

 object to the bottom of the cell. They must 

 be applied molten, and this is readily done 

 if the circular stage of the turntable is 

 heated while a small quantity of the 

 cement is melted in a capsule. The cement 

 is then apphed with a brush exactly as 

 though it were a varnish and permitted 

 to cool. These colored cement back- 

 grounds were widely used in the old days 

 and should receive more attention than is 

 at present the case. 



Selection of a Cement to Hold the Object 

 in a Cell 



This is the most difficult, as well as the 

 most important, of the decisions which 

 have to be made. A well-prepared dry 

 wholemount should not have any visible 

 cement obscuring the object, but the ob- 

 ject must at the same time be so firmly 

 held that it will stand the relatively rough 

 handling to which most slides are sub- 

 jected. If the sUde is to be mounted as a 

 transparent wholemount, there is nothing, 

 in the author's opinion, which can com- 

 pare with gum tragacanth; and a simple 

 dispersion of this gum in water, with the 

 addition of some preservative such as 

 thymol, is better than any of the more 

 complex formulas. Tragacanth has the 

 useful property of being transparent in 

 thin films, but these thin films are not 

 strong enough to hold objects larger than 

 diatoms or butterfly scales. To use this 

 gum one takes the slide on which the 

 selected cell has already been prepared 

 and turns, with the aid of a turntable, a 

 very thin uniform layer on the bottom of 

 the cell. The preparation of this thin uni- 

 form layer requires experience and skill 

 for which no description can substitute. 

 The adhesive is then allowed to dry and 



the objects are arranged on it in the re- 

 quired positions. As soon as all the objects 

 have been laid on the dry film, it is placed 

 in a moist, warm atmosphere wliich is usu- 

 ally secured by bending open-mouthed 

 over the shde and breathing very slowly 

 and carefully. This makes the layer of 

 tragacanth sticky so that the objects ad- 

 here; it will dry again in a few moments. 

 This was the method used to prepare 

 those pictures, made with the scales of 

 butterflies, or selected diatoms, which 

 used to be a feature of the catalogs of old- 

 time microscope preparers. There is, how- 

 ever, no reason why the method should 

 not be employed for scientific purposes, 

 for it is often desirable, particularly when 

 dealing with diatoms, to arrange them in 

 a selected pattern. Small objects of this 

 type may readily be placed in position if 

 they are picked up on the end of a hair 

 attached to a needle-holder; if they do not 

 stick to the hair at the first trial, it is only 

 necessary to moisten it with the hps. 



Gum tragacanth may also be used to 

 attach larger objects (such as Foraminif- 

 era and Radiolaria) to a paper background 

 but it will not stick satisfactorily to either 

 a resinous or wax surface. For these larger 

 objects it is necessary to take a very small 

 sable brush and place a drop of tragacanth 

 on the surface of the paper background 

 which has previously been moistened. The 

 individual object is then picked up and 

 pressed into the surface of the drop, which 

 is then allowed to dry. If the appearance 

 of the preparation is not very important a 

 fairly thick smear of the mucilage may be 

 placed all over the paper and the objects 

 sprinkled on; this gives, however, a clumsy 

 and unfinished appearance. 



The principal objection to the use of 

 optical-dead-black varnish as a back- 

 ground is that aqueous adhesives will not 

 adhere to it. Both gum arable and gum 

 tragacanth will stick for a certain length 

 of time, but the author has never known 

 a mount made with these adhesives in 

 which the object did not loosen within a 

 period of two or three months. Nothing is 

 more annoying than to take the trouble to 

 make a mount of ^elected foraminiferans 

 and then, a few months later, to find one 

 or two specimens rolling about inside the 



