USEFUL MICROSCOPE ACCESSORIES 125 



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 

 and it is to be regretted that sheets made from cellulose acetate 

 cannot be purchased in the open market of the same thickness 

 as those made from the nitrocellulose. 



Treatment of material with alkalies or at a high heat must 

 be confined to supporting slips made from platinum foil. In 

 fact, a small piece of platinum foil 15 to 20 mm. long by about 

 7 mm. wide, sufficiently thick to remain flat when heated at a 

 corner may be considered as a necessity. The foil must be 

 kept flat, clean and polished. Since it is opaque, the materials 

 must eventually be transferred to glass, quartz, or celluloid 

 slides for examination after having been subjected to the proper 

 reagent or heat treatment. When very low magnifications are 

 permissible it is possible to examine the material upon the plati- 

 num foil without transferring, the illumination being either by 

 oblique light or by some form of vertical illuminator. 



1 See also page 269. 



