28 BACTERIA IN RELATION TO PLANT DISEASES. 
on by severe reagents, when they may be stained in mordanted solutions or in dyes 
which have been preceded by a mordant. ‘The outer wall of the bacterium generally 
reacts to stains in the same way as the flagella, z. ¢., it usually remains unstained. 
Staining media may be roughly divided into four groups: (a) Simple stains 
dissolved in water, e. g., fuchsin (basic), gentian violet, methylene blue ; (4) alcoholic 
solutions and various complex stains, ¢. g., saturated alcoholic solutions of anilin 
dyes, alcohol-iodine, iodine potassium iodide, Russow’s cellulose test, Ziehl’s carbol- 
fuchsin, Loeffler’s alkaline methylene blue, Ehrlich’s anilin-water gentian violet, 
Gabbett’s stain, Gram’s method, Delafield’s haematoxylin, Ehrlich’s acid hzema- 
toxylin, Heidenhain’s iron-hzematoxylin, Fleming’s triple stain; (c) flagella and 
capsule stains, ¢. g., Loeffler’s stain, Moore’s modification, Fischer’s modification, 
Bunge’s stain, Léwit’s stain, van Ermengem’s nitrate of silver method, Zettnow’s 
gold method, etc.; (d) stains for spores, é. g., prolonged exposure to simple stains 
dissolved in water (which should 
have little effect), steaming carbol- 
fuchsin with methylene blue for 
contrast, etc. (see “‘ Formule ” and 
Bibliography of General Literature 
under “ Flagella,” “Spores,” etc., for 
various observations on staining). 
Griibler’s stains are preferred. 
Cover-glasses should be clean and 
free from fat, traces of which are 
easily removed in a Bunsen flame. 
A little experience is necessary in 
flaming thin covers in order not to 
crack them. ‘They may be passed 
through the flame three times, wait- 
ing a moment or two after each pass 
for them to cool. The student should 
see that the water used in making 
Fig. 27.* the cover-glass preparations or the 
stains does not itself contain bacteria. It is usually wise first to dry a drop of 
the water on the cover and stain without addition of the bacteria. Eternal vigi- 
lance is the price of trustworthy results. It is best to make all mounts on cover- 
glasses of a known and uniform thickness (0.15 mm.). Many a good preparation has 
been spoiled for examination with lenses of a short-working distance by mounting 
under a thick cover-glass, and sometimes the lens itself has been ruined in the 
attempt to focus. The thickness of covers often varies greatly from the statements 
of dealers, and they should not be accepted until tested with a reliable cover-glass 
measurer (fig. 27). 
*Fic. 27.—Zeiss cover-glass measurer. The cover in place shows a registered thickness of 0.18 
mm, Fractions of an inch are also registered on this instrument. 
a 
Tr 
ee 
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