248 SLIDES AND COVER-CLASSES [CH. IX 



been touched by the hands for a minute amount of oily substance leaves a 

 stratum on the slide which causes the liquids used to heap up instead of flow- 

 ing out perfectly flat. That is, the slide is wet with difficulty and the liquid 

 instead of forming a film tends to assume the spheroidal state. Sometimes 

 new gauze or other cloth used may not be wholly free from oily substance, or 

 the soap was not wholly eliminated in washing. Such wiping cloths will not 

 make the slides ready for good films. Some workers soak the gauze in sulfuric 

 ■ether to remove the last traces of oily substance. This is done more especially 

 in cleaning cover-glasses for films, see below. Burnett, p. 22, in speaking of 

 blood smears says : "The slides should be thoroughly clean. Unused slides 

 may be cleaned in strong soap or "gold dust" solution, well rinsed in water, 

 then placed in alcohol from which they are wiped and polished." 



I 333. Cover-Glasses or Covering Glasses. — These are circular or quad- 

 rangular pieces of thin glass used for covering and protecting microscopic 

 ■objects. They should be very thin, o.ioto 0.25 millimeter (see table, \ 32-34). 

 It is better never to use a cover-glass over 0.20 mm. thick, then the prepara- 

 tion may be studied with a 2 mm. oil immersion as well as with lower objec- 

 tives. Except for' objects wholly unsuited for high powers, it is a great mis- 

 take to use cover-glasses thicker than the working distance of a homogeneous 

 objective (§ 69). Indeed, if one wishes to employ high powers, the thicker 

 the section the thinner should be the cover-glass (see. \ 337). 



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

 which it is placed. 



Figs. 191-192. Figures of square and 

 of circular cover-glasses. (Cuts loaned by 

 the Spencer Lens Co. ) 



\ 334. 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 immediately. 



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 glass box or Petri dish. 



