THE USE OF STRONGER STAINING SOLUTIONS. 117 



have been precipitated for some reason or other oat 

 of the stain, sometimes their origin is unknown ; in 

 either case they may cause a considerable amount of 

 confusion, especially if the cover-glass has not been 

 left long enough in the decolourising solution, a mis- 

 take often made by the beginner. The difficulty may, 

 however, be easily overcome by placing the cover- 

 glass for fi-om five to ten minutes in a watery solution 

 of methylene blue; and then rinsing it well with 

 water. If an examination with the microscope be 

 now made, it will be found that whilst the tubercle 

 bacilli retain unchanged their deep red colour^ all 

 the other objects, nuclei, pus corpuscles, and other 

 bacteria are stained a more or less deep blue. If one 

 of the doubtful objects again presents itself, we see 

 that now it differs in colour as well as in form from 

 the tubercle bacilli, its original red colour being 

 changed into a bluish violet, whilst the bacilli remain 

 unaltered. It may, however, occur that amongst the 

 tubercle bacilli there are some which appear to be 

 quite different in form from the others. This is caused 

 by the fact that sometimes they are so closely packed 

 over each other that it is impossible to recognise the 

 individual forms. 



An aniline water solution of gentian, or methyl 

 violet, may be used instead of the aniline fuehsine ; 

 only then saturated aqueous solutions of vesuvin, or 

 of Bismarck brown, must be used for the contrast 



