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. 
Fic. 11.—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-celis, 
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 
