24 CLINICAL BACTERIOLOGY AND H/EMATOLOGY 



are resting on the table. In using them put the cover-glass 

 between the jaws of the forceps, which must clip them a little 

 distance from the corners, otherwise the stain will run into the 

 forceps. Make a point of placing them on the table with the 

 keyhole upwards, so that you will always know on which side 

 of- the cover-glass the film has. been spread. But the staining 

 can be carried 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. 



It is not absolutely necessary to use cover-glasses in the 

 preparation of these bacterial films unless they are to be kept 

 for future reference, and except in such cases I have discarded 

 them for years. To examine the unmounted film it is only 

 necessary to dry it thoroughly, and to put the immersion oil 

 (see p. 28) directly on to the stained area. This method is 

 especially useful in searching for the tubercle bacillus, for no 

 one wants to keep films of tuberculous sputum after the diag- 

 nosis has been made, and if the necessity does arise a drop of 

 balsam can be placed on the film (without removing the oil) 

 and the cover-glass applied. The one advantage of covering 

 the films at once is that it permits of the use of the low powers, 

 and as this should always be done by beginners, the covering 

 of the preparations has been recommended throughout. 



GRAM'S METHOD OF STAINING 



The method of staining described above is available for 

 most organisms, and therein consists its advantage. But 

 other things than bacteria are stained : pus cells, fragments 

 of tissue, debris, etc., 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 structures (with the exception of par- 

 ticles 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 



