EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 37 



omitted entirely if it is desired. A suitable contrast-stain 

 after fuchsin staining is a solution of methyl ene-blue; after 

 gentian-violet staining, Bismarck brown. 



Those who have had experience in staining tubercle bacilli 

 soon discover that the bacilli exhibit some differences in their 

 resisting power to strong acids. One encounters occasionally 

 bacilli that are perfectly stained side by side with others that 

 are more or less completely decolorized. These facts show the 

 necessity of practice with any method, and of exercising caution 

 and judgment in making a diagnosis where the number of 

 bacilli happens to be scanty. If tubercle bacilli are not found 

 in the first preparation, other preparations should be made. 

 Sometimes a large number of cover-glasses must be examined. 



Various expedients have been devised to concentrate tubercle 

 bacilli when only a small number may be present in a sample 

 of sputurn. In Biedert's method about 15 c.c. of sputum are 

 mixed with 5 c.c. of distilled water, 4 to 8 drops of sodium 

 hydrate solution are added, and the mixture is bailed. After 

 boiling, about 15 c.c. of distilled water are added. The 

 mixture may be set aside in a conical glass for from twenty- 

 four to forty-eight hours to allow sedimentation or the sedi- 

 ment may be precipitated rapidly by the use of the centrifuge. 

 In either case cover-glass preparations are made from the 

 sediment in the manner already described. The sediment 

 will be found to have little adhesive power, and will not stick 

 well to the cover-glass. It is convenient to save some of the 

 original sputum and mix it with the sediment for this purpose. 



Fixation of Tissues. — Pieces of organs about i cm. in 

 thickness may be taken. Alcohol is the best agent for pre- 

 serving them. The fixation will be completed in a few 

 days. It is best to change the alcohol. The amount of the 

 alcohol must be twenty times the bulk of the tisuse to be pre- 

 served. 



Ten parts of the standard 40 per cent, solution of formalde- 



