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 employment save in investigations 

 where glass or platinum foil cannot possibly be used. 



For use with hydrofluoric acid and its salts object slides of 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. 1 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. 



