CH. Vn.~\ SLIDES AND COVER-GLASSES. 141 



handkerchief, usually fits them for ordinary use. If they are not satis- 

 factorily cleaned in this way, soak them a short time in 50% or 75$ 

 alcohol, let them drain for a few moments on a clean towel or on blot- 

 ting paper, and then wipe with a soft cloth. In handling the slides 

 grasp them by their edges to avoid soiling the face of the slide. After 

 the 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 gum- 

 my substance has been used upon the slides, they may be freed from the 

 balsam, etc., by soaking them for a week or more in one of the clean- 

 ing mixtures for glass. If they are first soaked in xylene, benziu or tur- 

 pentine to dissolve the balsam, then soaked in the cleaning mixture, the 

 time required will be much shortened (§ 227). After all foreign mat- 

 ter 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. 



If slides with large covers, as in mounted series, are put into the 

 cleaning mixture, the swelling of the balsam is liable to break the covers. 

 Dissolving away the balsam with turpentine, etc., avoids this, and 

 greatly shortens the time necessary for cleaning the old slides and covers. 



Another excellent method for balsam mounts is to heat the slides until 

 the balsam is soft and then remove the cover-glasses. The cleaning 

 mixture can then act on the entire surface. It should be said, however, 

 that at the present price of slides and cover-glasses it is hardly worth 

 while to clean those that have been used in balsam mounting. 



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

 or quadrangular pieces of thin glass used for covering and protecting 

 microscopic objects. They should be very thin, j^ to -fo% millimeter 

 (see table, § 27). It is better never to use a cover-glass over y 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 (§47). Indeed, if 

 one wishes to employ high powers, the thicker the sections the thinner 

 should be the cover-glass (see § 235). 



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

 which it is placed. 



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

 into a glass dish of some kind containing one of the cleaning mixtures 



