CH. IX] 



SLIDES AXD COVER-GLASSES 



249 



Handle them always by their edges, or use fine forceps. Do not put the fingers 

 on the faces of the covers, for that will surely cloud them. 



335. Cleaning Cover-Glasses for Special Uses. — As with slides, covers 

 intended for films or other purposes where the last particles of oily substance 

 must be removed, are best put one by one into dichromate cleaning mixture 

 (J 339). After a day or more this is poured off and a stream of fresh water 

 allowed to run on the covers until all the cleaning mixture is removed. Then 

 distilled water is added and allowed to stand a few minutes. This is poured 

 off and S 2 ' ', , orgs 11 ,, alcohol added. The covers remain in this until needed. 

 In wiping use the precautions given with slides (jj 332). 



Figs. 193-194. Glass box and Petri dish for clean 

 cover-glasses. [Cuts loaned by the Whitall Tatum Co.). 



\ 336. Cleaning Large Cover-Glasses. — For serial sections and especially 

 large sections, large quadrangular covers are used. These are to be put one 

 by one into a cleaning mixture as for the smaller covers and treated in every 

 way the same. In wiping them one may proceed as for the small covers, but 

 special care is necessary to avoid breaking them. It is desirable that these 

 arge covers should be thin — not over 0.15-0.20 mm. otherwise high objectives 

 cannot be used in studying the preparations. 



Fig. 193. Micrometer Calipers {Brown and Sharpe). Pocket Calipers, 

 graduated in inches or millimeters, and well adapted for measuring cover- 

 glasses. 



\ 337. Measuring the Thickness of Cover-Glasses. — It is of great advan- 

 tage to know the exact thickness of the cover-glass on an object; for, (a) in 

 studying the preparation one would not try to use objectives of a shorter work- 

 ing distance than the thickness of the cover ( \ 69); (b ) In using adjustable 

 objectives with the collar graduated for different thicknesses of cover, the 



