APPARATUS 
17 
Slides and covers are a constant expense. Many slides now 
upon the market are imperfect. Beware of slides which are not 
perfectly flat. Be skeptical in regard to any claim that slides are 
already clean enough to use. Of course, there should be no bubbles. 
“ White” slides are to be preferred to those which appear greenish in 
the box. For ordinary class work, slides of medium thickness are 
more serviceable, but for critical cytological work many investigators 
prefer very thin slides. 
There is never any objection to very thin covers, except that they 
require care in cleaning. For mounts which are to be used with an 
immersion lens, it is better to have the cover of the same width as 
the slide. The advantage is evident, since there is no danger of 
getting balsam on the cover when wiping off the immersion fluid; 
besides, one can put sections to the very edge of the slide and still 
be sure that they will be covered. Since most mounts for research 
work are mounted under long covers and are intended for examina¬ 
tion with immersion lenses, we should recommend covers of 25X50 
mm., or even 25X60 mm. Round covers are desirable only when 
mounts are to be sealed on a turntable. Larger slides and corre¬ 
spondingly larger covers are needed for special purposes. 
By consulting a catalogue, which will be furnished by any dealer, 
the beginner can determine what he needs to buy, and what he can 
find substitutes for, if it is necessary to be very economical. 
