62 A MANUAL OF BACTERIOLOGY 



ances carefully observed under Zeiss' B, C, and D 

 objectives, or any similar low powers. After this a 

 cover-glass preparation should be made by rubbing 

 a needle, previously dipped into the growth on the 

 plate, on a clean cover-glass. A drop of sterilised 

 water is now added; the cover-glass is allowed to 

 dry ; then passed three times through the Bunsen 

 flame ; and finally stained with a drop of fuchsine 

 or some other aniline colour. The cover-glass pre- 

 paration should be temporarily or permanently 

 mounted, according to the methods described later 

 in this chapter. After mounting, the preparation 

 should be examined under high powers, such as 

 Zeiss' J and T ^. It should be borne in mind that 

 the eye has to be trained in order to see objects 

 distinctly with such high powers ; but, it may be 

 remarked, that ' in all extremely delicate work with 

 high-power lenses, the first difficulty is the greatest. 

 If once an object has been seen, however difficult, it 

 is immensely easier to see it again. On the other 

 hand, there is as great a diversity in different 

 individuals in the sensitiveness of the retina, as there 

 is in the sensitiveness of the olfactory, or auditory 

 nerves. It is impossible to enable some persons to 

 see objects beyond a certain limit of minuteness ; as 

 it is to enable others to detect certain scents, or hear 

 notes pitched higher or lower than a given point.' 



The fractional cultivation method consists in the 

 attempt to isolate, by successive cultivations, the 

 different organisms that have been growing pre- 

 viously in the same culture. A number of tubes 

 containing various cultivation media (sterilised) are 



