CH. VII] SLIDES AND COVER-GLASSES 163 



to injure the slides or covers. After one day or longer, pour off the cleaning mix- 

 ture 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. 



FIGS. 129-130. Figures of square and of circular cover-glasses. See also Fig. 

 162 for covers on serial sections. 



\ 238. Wiping the Cover- Glasses. When ready to wipe the cover-glasses, 

 remove several from the alcohol and put them on a soft, dry cloth, or on some of 

 the lens or filter paper to let them drain. Grasp a cover-glass by its edges, cover 

 the thumb and index finger of the other hand with a soft, clean cloth or some of the 

 the lens paper. The bleached cheese cloth ( \ 235 ) is good for wiping covers. Grasp 

 the cover between the thumb and index and rub the surfaces. In doing this it is 

 necessary to keep 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 well wiped, 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 clear 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. 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. 



FIG. 131. Glass dish for holding covers 

 ( Whitall, Tatum & Co.}. 



\ 239. Cleaning Large Cover-Glasses. For 

 serial sections and especially large sections, 

 large quadrangular covers are used (Fig. 162). 

 These are to be put one by one into a cleaning 

 mixture as for the smaller covers and treated in 



every way the same. In wiping them one may proceed as for the small covers, 

 but special care is necessary to avoid breaking them. It is especially desirable 

 that these large covers should be thin not over 0.15-0.20 mm. otherwise high ob- 

 jectives cannot be used in studying the preparations. 



$ 240. Measuring the Thickness of Cover-Glasses. It is of the greatest 

 advantage to know the exact thickness of the cover-glass on an object ; for, (a) 



