THE MICROSCOPE. 43 



i. A knowledge of the effects of re-agents implies nothing 

 less than an acquaintance with the majority of histological 

 methods. In the course of the following demonstrations 

 the student will more correctly appreciate the effects of 

 re-agents by bearing in mind that they are applied to the 

 tissues for the purpose of ascertaining i. Their chemical 

 composition, e.g. fat is blackened by osmic acid, and 

 dissolved by ether ; starch is coloured blue by iodine ; 

 earthy salts are dissolved by mineral and various other 

 acids ; albuminous tissues are rendered transparent, and 

 may be partially dissolved by acetic acid. 2. Their 

 structural composition. Many textures are so transparent 

 and their outlines so faint, that great assistance in deter- 

 mining their structure is obtained by staining them with 

 various substances : e.g. a solution of nitrate of silver, by 

 darkening the outlines of epithelial cells, brings into view 

 what is otherwise invisible. Chloride of gold, by rendering 

 nerve fibrils of a violet colour, often reveals them where 

 they cannot otherwise be seen ; carmine, magenta, and 

 logwood, by staining nucleated protoplasts, are of the 

 greatest service in showing their boundary lines. Also 

 such substances as acetic acid, glycerine, and oil of cloves, 

 by rendering transparent some parts .of the tissues, permit 

 of others being seen. 3. Some re-agents are applied for 

 the purpose of studying their effects on vital phenomena. 

 With this view heat, electricity, and various chemicals, are 

 applied to corpuscles that exhibit movement e.g. white 

 blood corpuscles, cilia, etc. 



58. Delineation of the Object. It is very important 

 that the student should embrace every opportunity of 

 making drawings of microscopical objects, for drawing 

 necessitates a thorough inspection of an object, and it 

 impresses its features deeply upon the memory. The 

 smooth but not too highly glazed "antique note paper" 

 made by Pirie and Son, Aberdeen, is an excellent paper 

 for drawing. The pencils used are H.B., 4 H., and 6 H. 

 The shading should not be produced by lines, as in 

 ordinary drawing and wood-engraving, but it should be a 

 continuous shade produced by a rapid to and fro move- 



