67 MOUNTING AND LABELING. 



slides are cleaned, they should be stored in a place as free as possible 

 from dust. 



For used slides, if only water, glycerin or glycerin jelly has been 

 used on them, they may be cleaned with water, or preferably, warm 

 water and then with alcohol if necessary. Where balsam or any oily 

 or gummy substance has been used upon the slides they may be best 

 freed from the balsam, etc., by soaking them for a week or more, in one 

 of the cleaning mixtures for glass (§ 164). After all foreign matter is 

 removed the slides should be very thoroughly rinsed in water to remove 

 all the cleaning mixture. They may then be treated as directed for 

 new slides. 



§ 160. Cover-Glasses or Covering Glasses. — These are circular or 

 quadrangular pieces of thin glass used for covering and protecting mi- 

 croscopic objects. They should be very thin, y 1 ^ to y^- millimeter 

 (see table, § 17). It is better never to use a cover glass over T 2 ^ 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 (§ 38). 



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

 over which it is placed. 



§ 161. Cleaning Cover-Glasses. — New cover-glasses should be put 

 into a glass dish of some kind containing one of the cleaning mix- 

 tures (§ 164) and allowed to remain a day or longer. In putting them in, 

 push one in at a time and be sure that it is entirely immersed, otherwise 

 they adhere very closely, and the cleaning mixture is unable to act 

 freely. Soiled covers should be left a week or more in the cleaning 

 mixture. An indefinite sojourn in the cleaner does not seem to injure 

 the slides or covers. After one day or longer, pour off the cleaning 

 mixture into another glass jar, and rinse the cover glasses, moving them 

 around with a gentle rotary motion. Continue the rinsing until all the 

 cleaning mixture is removed. One may rinse them occasionally, and 

 in the meantime allow a very gentle stream of water to flow on them or 

 they may be allowed to stand quietly and have the water renewed from 

 time to time. When the cleaning mixture is removed rinse the covers 

 well with distilled water, and then cover them with 50% to 75% alcohol. 

 Wiping the cover-glasses. — When ready to wipe the cover-glasses re- 

 move several from the alcohol and put them on a soft dry cloth or on 

 some of the Japanese paper, to let them drain. Grasp a cover-glass by 

 its edges, cover the thumb and index of the other hand with a soft 

 clean cloth or some of the Japanese paper. Grasp the cover between 

 the thumb and index and rub the surfaces. In doing this it is neces- 

 sary to keep the thumb and index well opposed or on directly opposite 



