THE USE OF STRONGER STAINING SOLUTIONS. 119 



most strongly advised to entirely disregard all the 

 other methods which, have been recommended, al- 

 though, no doubt, they are more convenient, and in 

 the majority of cases yield good results ; yet they are 

 unreliable, and just in the critical cases may mislead 

 him. Mention is here made of the one most com- 

 monly used, in order to point out wherein it fails, and 

 its unfortunate consequences. Many people put the 

 methylene blue into the acid solution, in order to de- 

 colourise and restain at the same time. This has many 

 disadvantages. The preparation is hidden by the 

 dye, and so it is very difficult to know just when 

 sufficient decolourisation has taken place, and thus 

 much depends en chance. In consequence, the de- 

 colourisation is generally either insufficient, in which 

 case the whole preparation is stained violet instead of 

 blue, and therefore the contrast between the tubercle 

 bacilli and the rest of the preparation is diminished ; 

 or, as is more frequently the case, it is too complete, 

 and even the tubercle bacilli have lost some of their 

 colour. For even the tubercle bacilli cannot resist 

 the prolonged action of acids, the difference shown by 

 diflferent bacteria in their powers of retaining stain 

 being one of degree only. Moreover, all tubercle 

 bacilli are not equally affected by the acid ; some might 

 be completely decolourised, and others only partially 

 so. This would, of course, render their recognition 

 in sputum far more difficult; and yet there is another 



