68 MOUNTING AND LABELING. 



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 seen. 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 paper 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. 



§ 162. Cleaning Large Cover-Glasses. — For serial sections and 

 especially large sections, large quadrangular covers are used. These 

 are to be put one by one into 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. A safe and good way to clean the large covers is to take two 

 perfectly flat, smooth blocks considerably larger than the cover- 

 glasses. These blocks are covered with soft clean cloth, or with sev- 

 eral thicknesses of the Japanese paper ; if now the cover-glass is placed 

 on the one block and rubbed with the other the cover may be cleaned 

 as by rubbing its faces with the cloth covered finger and thumb. 



§163. 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) One would not try to use objectives in studying the 

 preparation of a shorter working distance than the thickness of the 

 cover (§ 38) ; (b) In using adjustable objectives with the collar gradu- 

 ated for different 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 9 (see table § 17). Furthermore, if there 

 is a variation from the standard, one may remedy 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 (§ 63). 



In the so called No. 1 cover-glasses of the dealers in microsopical 

 supplies, the writer has found covers varying from -^ mm. to ^ 

 mm. To use cover-glasses of so wide a variation in thickness without 

 knowing whether one has a thick or a thin one is simply to ignore the 

 fundamental principles on which correct microscopic images are ob- 

 tained. 



It is then strongly recommended that every preparation shall be cov- 

 ered with a cover-glass whose thickness is known, and that this thick- 

 ness should be indicated in some way on the preparation. 



