l62 



SLIDES AND COVER-GLASSES 



[CH. VII 



Fig. 128. Glass slide or slip of the ordinary size for microscopic work (3x1 in., 

 76 x 25 mm.). {Cut loaned by the Spencer Lens Company). 



For old 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 an} - oily or gummy 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 cleaning mixtures for glass. If they are first soaked in 

 xylene, benzin or turpentine to dissolve the balsam, then soaked in the cleaning 

 mixture, the time required will be much shortened (f 242). After all foreign 

 matter is removed the slides should be 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, avoids this, and greatly shortens the time neces- 

 sary 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 turpentine cleaning mix- 

 ture, etc., can then act on the entire surface. It should be said, however, that at 

 the present price of slides and cover-glasses it costs nearly as much as the slides 

 and covers are worth to clean those that have been used in balsam mounting. 



(S 236. Cover-Glasses or Covering Glasses. — These are circular or quadr- 

 angular pieces of thin glass used for covering and protecting microscopic objects. 

 The}' should be very thin, o. 10 to 0.25 millimeter (see table, 1 29). It is better 

 never to use a cover-glass over 0.20 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 ( £ 61). Indeed, if one 

 wishes to employ high powers, the thicker the sections the thinner should be the 

 cover-glass (see J 240). 



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

 which it is placed. 



'i 237. Cleaning Cover-Glasses. — New cover-glasses should be put into a 

 glass dish of some kind containing one of the cleaning mixtures (£ 242) and al- 

 lowed to remain a day or longer. In putting them in, push one in at a time and 

 be sure that each 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 



