32 BACTERIOLOGICAL DIAGNOSIS. 



out equally well in a watch-glass, and the cover-glass 

 may be held in dissecting forceps. 



It is far easier and more satisfactory in every way to 

 make the films on the slides. Beginners will find that 

 they will break large numbers of cover-glasses (which 

 must be thin), drop more on the floor, and will be in 

 constant doubt as to which is the film side. With 

 slides these difficulties do not occur, and the use of 

 forceps is quite unnecessary. 



FIG. ii. Cornet's Forceps. 



GRAM'S METHOD OF STAINING. 



The method of staining described above is available 

 for all organisms, and therein consists its advantage. 

 But other things than bacteria are stained ; pus-cells, 

 fragments of tissue, debris, &c., will all be coloured, 

 and may obscure, or even be mistaken for, bacteria. 

 Gram's method possesses the enormous advantage that 

 by its use the bacteria are coloured, while other struc- 

 tures (with the exception of particles of keratin and 

 dividing nuclei) are not. Hence in a film stained in 

 such a way the bacteria are very distinct. 



Gram's method possesses another advantage. It is 

 a selective stain. Some bacteria retain the stain, whilst 

 others do not, and this fact is of great value in dia- 

 gnosis. The diphtheria bacillus, for instance, stains 



