METHOD OF EXAMINING CULTURES 25 



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. 29) 

 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 diagnosis 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 recom- 

 mended 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 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 



