OBJECT SLIDES 
151 ' 
short lengths and to mark one side in some manner and always 
apply the hands to the marked side only. Frequent laundering 
is essential. 
The difficulty of preparing absolutely clean slides is never 
fully appreciated until one has tried working with dark-ground 
illuminators and various types of the ultramicroscope. In the 
more refined methods of ultramicroscopic investigations it is 
found that glass slides cannot be made sufficiently free from 
objectionable surface films for use, and recourse must be had to 
quartz slides or disks which, after cleaning as described above, 
are heated to bright redness just prior to being employed. 
Quartz (fused silica) slides may now be obtained from any 
firm dealing in this material, sufficiently free from air bubbles, 
to permit using even high powers, and of such transparency as 
to leave little to be desired. Small slips or tiny cells of silica 
will be found most useful where corrosive acid chemicals are 
employed or where the material must be heated to a temperature 
somewhat higher than the fusing point of glass. In the investi¬ 
gation of ultramicroscopic particles or in observations upon the 
action of ultraviolet light, fused silica supports and covers are 
essential. The price of silica object slides is still so high, however, 
as to be prohibitive to their emplojonent save in investigations 
where glass or platinum foil cannot possibly be used. 
For use with hydrofluoric acid and its salts object slides oi thin 
celluloid will be found practicable and far more convenient than 
glass slides varnished or coated with Canada balsam. In the 
absence of good celluloid slips, glass object slides may be coated 
with a thin film of “ Zapon ” or “ Bakelite ” varnish.^ Although 
celluloid may now be obtained sufficiently clear and colorless for 
all the usual microchemical methods involving tests with fluor¬ 
ides it possesses the drawback of great inflammability and since 
most of these tests require a gentle heat for their proper develop¬ 
ment, exceeding great care is necessary to avoid the complete 
destruction of the slide and preparation during heat treatments. 
Object slides made from ‘‘ fireproof ” photographic films of cellu¬ 
lose acetate are therefore better than slips of ordinary celluloid 
1 See also page 317. 
