XXll INTRODUCTION 



Picro-carmine has the advantage of being an aqueous 

 stain, and may therefore be used either for fresh or hardened 

 tissues. Immersion of the specimen in acid-alcohol for some 

 time after staining improves the effect. 



Haematoxylin (Kleinenberg's solution) gives excellent 

 results if used with due care, but a trace of chromic or other 

 acid may completely destroy the colour after the specimen is 

 mounted. 



Osmic acid, of which a 1 per cent, or ^ per cent, aqueous 

 solution is used, is both a hardening and a staining reagent. 

 It is only used with fresh tissues, and only with small speci' 

 mens. It kills Protozoa instantly, and after a time stains 

 their nuclei, though only faintly. Objects hardened with 

 osmic acid may be stained with picro-carmine. A mixture of 

 chromic and osmic acids forms a useful hardening reagent. 



Acetic acid, 1 per cent, solution, is useful for render- 

 ing the nuclei of cells more distinct : it is used with fresh 

 specimens. 



VII. ON MICROSCOPICAL EXAMINATION. 



The microscope affords the means of investigating the 

 structure of minute animals, and the finer details of those 

 of larger size. The microscopical examination of the special 

 organs of the larger animals is of great importance, and must 

 on no account be neglected. 



The microscope consists of a body and a stand. The body 

 is a tube of metal carrying the lenses, which are the essential 

 part of the instrument. 



The stand supports the body, in such a way that the 

 latter may be moved up and down without any lateral move- 

 ment. It also supports a stage, which ought to be horizontal, 

 to hold the object to be examined. The stage has a round 

 hole in its centre, through which light may be reflected on to 

 the object by a mirror, which is fixed to the stand below the 

 stage. This aperture is usually much larger than is necessary ; 

 but the excess of light, and especially the light far from the 

 optical axis of the lenses, is cut off by means of diaphragms. 



