I.] MICROSCOPIC EXAMINATION. 9 



well in alcohol so as to remove the dye from all except 

 the bacteria, but do not wash with alcohol too long, then 

 rinse in distilled water, dry and mount. 



The most useful dyes in the examination of animal tissues 

 for bacteria are those aniline dyes that are soluble in water ; 

 these are preferable to those soluble in alcohol only. They 

 have all great affinity for cell nuclei (Hermann) and belong 

 to the group of neutral or basic aniline colours. Methyl- 

 blue, methyl-violet, vesuvin, Bismarck -brown, magenta, 

 fuchsin, gentian-violet, Spiller's purple, rosaniline, Hum- 

 boldt's red (purple), are the dyes most commendable. 



For staining of cover-glass specimens, as well as for sections 

 made of fresh tissues, the above dyes can be advantageously 

 used in the following manner : 2 to 5 grammes of the solid 

 dye are rubbed up in a mortar with 10 ccm. of absolute 

 alcohol ; add then gradually, while mixing, warm distilled 

 water, to bring up the total to 100 ccm. ; filter and keep in 

 stoppered bottle. For use, filter a little of the dye into a 

 watch-glass. For staining sections of tissues that have been 

 hardened, the above dyes prepared with Weigert's aniline 

 oil are preferable; they are prepared thus: (a) Make a 

 saturated watery solution of pure aniline (aniline oil) by 

 mixing in a bottle one part of aniline oil with three parts 

 of distilled water ; shake well every half hour for four to 

 six hours, decant the water as the oil settles to the bottom. 

 The decanted fluid is the saturated watery solution of aniline. 

 Of this take 100 ccm. Add to this (b) a saturated alcoholic 

 solution of fuchsin, magenta, gentian-violet, Humboldt's 

 red, methyl-blue or methyl-violet, n ccm. ; mix well, filter 

 into stoppered bottle. The sections are left in this dye for 

 from a few minutes to several hours (Humboldt's red 

 requires only a few seconds). Different bacteria require 

 different periods to stain. As a rule warming the dye 



