126 



are wiped, put them in a clean glass box. 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. 



§ ii6. Measuring the thickness of the cover-glasses. 

 With the cover-glass measurer determine the thickness of the cover- 

 glasses and sort them into three groups : (a) those with a thick- 

 ness of .13-. 17 mm., (^) those less than .13 mm., and (c) those 

 thicker than .17 mm. Groups (a) and {b) only should be used ; {c) 

 should be discarded or used only with objects for low magnification. 



It is adva:ntageous to know the thickness of the cover-glass on 

 an object for the following reasons : (a) That one do not try to use 

 objectives in studying the preparation of a shorter working distance 

 than the thickness of the cover-glass ( Microscopical Methods, §57) ; 

 (i) In using adjustable objectives with the collar graduated for dif- 

 ferent thicknesses of cover, the collar might be set at a favorable 

 point without loss of time ; (c) For unadjustable objectives the 

 thickness of cover may be selected corresponding to that for which 

 the objective was corrected (see Microscopical Methods § 27 table). 

 Furthermore if there is a variation from the standard one may rem- 

 edy it in part at least by lengthening the tube if the cover is thinner 

 and shortening it if the cover is thicker than the standard (Micr. 

 Meth. § 96). 



§ 117. Cleaning slides. Rinse new slides thoroughly in 

 clean water and then wipe them with a soft towel. In cleaning the 

 slides handle them by their edges to avoid soiling the face of the 

 slide. After the slides are cleaned, to keep them free from dust 

 store them again in the box in which they came, or in a covered 

 glass dish or jar. 



§ 118. Cleaning used slides and covers. If only water, 

 glycerin or glycerin-jelly has been used on them, they may be cleaned 

 with water, preferably warm water, and then, if necessary, wiped 

 out of 50% alcohol. If balsam has been used, heat the slides until 

 the balsam is soft and then remove the cover-glasses. Scrape from 

 the slides all the balsam possible and place them (and cover-glasses) 

 in cleaning mixture for several days. If then they cannot be readily 

 cleaned, place them in fresh cleaning mixture for a period of several 

 days. Wash away the cleaning mixture thoroughly with water and 

 wipe them with a clean towel. 



