CH. X] CLEANING SLIDES AND COVER-GLASSES 317 



meter. It is better never to use a cover-glass over o. 20 mm. thick, then 

 the preparation may be studied with a 2 mm. oil immersion as well as 

 with lower objectives. Except for objects wholly unsuited for high 

 powers, it is a great mistake to use cover-glasses thicker than the 

 working distance of a homogeneous objective ( 76). Indeed, if one 

 wishes to employ high powers, the thicker the section the thinner 

 should be the cover-glass. 



The cover-glass should always be considerably larger than the object 

 over which it is placed. 



492. Cleaning cover-glasses for ordinary use. Covers may 

 be cleaned well by placing them in 82 % or 95 % alcohol containing 

 hydrochloric acid one per cent. They may be wiped almost imme- 

 diately. 



Remove a cover from the alcohol, grasping by the edge with the left 

 thumb and index. Cover the right thumb and index with some clean 

 gauze or other absorbent cloth; grasp the cover between the thumb 

 and index and rub the surfaces, keeping the thumb and index well 

 opposed on directly opposite faces of the cover so that no strain will 

 come on it, otherwise the cover is liable to be broken. 



When a cover is dry hold it up and look through it toward some 

 dark object. The cover will be seen partly by transmitted and 

 partly by reflected light, and any cloudiness will be easily detected. 

 If the cover does not look clear, breathe on the faces and wipe again. 

 If it is not possible to get a cover clean in this way it should be put 

 again into the cleaning mixture. 



As the covers are wiped put them in a clean shell vial (fig. 196), 

 glass box or Petri dish. 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. 



493. Cleaning cover-glasses for special uses. As with slides, 

 covers intended for films or other purposes where the least particles of 

 oily substance must be removed, are best put one by one into dichro- 

 mate cleaning mixture ( 497). 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 82 % or 95 % 



