27 



colour effect. The discovery of staining methods has been of 

 immense importance to microscopical science, resulting in the opening 

 up of the science of bacteriology. Stains may be divided into two 

 classes — general and selective. The former colour the whole of the 

 specimen, and are only useful in exhibiting structure too transparent 

 to be seen otherwise. The latter, as their name implies, colour only 

 certain kinds of tissue or cells ; thus it is possible in a single speci- 

 men to stain one portion blue, another red, etc., according to the 

 affinity each structure has for its particular stain. The action of some 

 stains is stopped before the surrounding structure is affected ; in 

 others the whole of the section is allowed to stain and afterwards 

 treated with acid or alcohol, in order to wash out the stain from those 

 structures which give it up most readily. There is no doubt that in 

 the future, as in the past, advance in microscopical science will be 

 largely indebted to the perfection of knowledge in the direction of 

 staining processes. 



Mounting. 



There is plenty of scope for the microscopist to exhibit his skill 

 and neatness in the final operations of mounting and finishing the 

 specimen. There are two kinds of mounts — opaque and transparent. 

 The treatment of opaque or dry objects is very simple, as they require 

 but little preparation. The chief precaution to observe is that the 

 object and the cell in which it is placed are thoroughly dry before 

 closing. Specimens of seeds, pollen, algoe, mosses, micro-fungi, 

 crystals, etc., are frequently mounted in an asphalte cell, being fixed 

 thereon with a tiny drop of gum. Cells are built up in various ways ; 

 they may be obtained from the optician in the form of rings of 

 ebonite, glass, tin, etc., of various sizes and thickness. These are 

 cemented to the glass slip with a thin ring of gold size. For this 

 purpose a " turntable " is required. This is a circular metal plate of 

 about 3^ inches diameter, mounted so as to revolve on a centre pin. 

 One or more concentric rings are engraved on its surface, and the glass 

 slip placed thereon and held in position with two brass clips. The 

 plate is then revolved, a small sable brush is charged with gold size or 

 other cement, and a circle made with it on the glass slip or cover. A 

 disc of asphalte varnish or brunswick black may be made in the same 

 way. Asphalte varnish is made by dissolving asphalte in benzole to a 

 proper consistence, adding a few grains of indiarubber to obviate 

 brittleness. Varnish cells may be made with the turntable of the desired 

 depth by ringing on successive coats of dammar varnish or gold size. 

 Dammar varnish is a thick solution of picked gum dammar in benzole. 

 Each coat should be allowed to dry before the application of a second. 

 When the last coat is in a slightly sticky condition the cover-glass is 

 gently attached and the slide finished by a coat of asphalte varnish just 

 covering the edge of the cover-glass. Some microscopists use Aspi- 

 nall's enamel for this purpose, others the well-known zinc cement. 



