PREPARATION OP STAINING SOLUTIONS 69 



remove the diaphragms, and to examine with the full power 

 of the Abbe's illuminator. 



The coloured image is best if the structural image be 

 effaced by rendering the shadows of the unstained parts 

 invisible in the broad cone of light. 



It must further be remarked regarding the structural 

 image, that the diaphragm should have the narrowest 

 possible aperture with a low power, but should increase in 

 size as higher powers are employed. 



Preparation of staining solutions. — For the simplest kind 

 of staining of bacteria solutions of fuchsine, methyl blue, 

 gentian violet, Bismarck brown, vesuvine (in equal parts of 

 water and glycerine), and methyl violet are used. Gentian 

 violet and fuchsine stain quicker and more intensely than 

 the others. In order to increase the staining power with 

 the various micro-organisms, certain mordants are employed, 

 as ifn other histological methods of staining. Aniline oil and 

 phenol are the mordants most used in bacteriological re- 

 search. The former, which is not a true oil, is obtained 

 from coal tar, and it is used for preparing an aniline tvater 

 in which the dyes, especially gentian violet or fuchsine, are 

 dissolved. The aniline water should be prepared freshly 

 each time, or in any case should not be allowed to stand 

 long, as it rapidly decomposes. To make it a test-tube 

 is filled with water which is shaken up with 1 to 2 c.cm. 

 of aniline oil until an emulsion is formed, which is 

 filtered. The clear filtrate is aniline water ready for use, and 

 enough of the alcoholic solution of the dye is then added 

 to render the liquid of a dark colour. The concentration 

 of aniline water amounts to 5 to 100. 



Trenkmann prepares his aniline water solution of gen- 

 tian violet in the following way : — A drop of a concentrated 

 alcoholic solution of gentian violet is let fall into a test- 

 glass and 10 c.cm. of water are added. Half of this is theii 



