CH. VI I] 



SLIDES AND COIER CLASSES. 



143 



perfectly flat, smooth blocks, considerably larger than the cover-glasses. 

 These blocks are covered with soft clean cloth, or with several thick- 

 nesses of the lens paper ; if now the cover-glass is placed on the one 

 block and rubbed with the other, the cover may be cleaned as b}' rub- 

 bing its faces with the cloth-covered finger and thumb. It is especially 

 desirable that these large covers should be thin — not over y 1 -^ to y 2 ^- 

 nnn, — otherwise high objectives cannot be used in studying the prepa- 

 rations. 



§ 225. 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 

 (§ 57) ! (b) In using adjustable objectives with the collar graduated 

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

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

 ness of cover may be selected corresponding to that for which the object- 

 ive was corrected (see table, § 27). Furthermore, if there is a varia- 

 tion from the standard, one may remedy it, in part at least, by length- 

 ening the tube if the cover is thinner, and shortening it if the cover is 

 thicker than the standard (§ 96). 



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

 supplies, the writer has found covers varying from T y T mm. to T %\ mm. 

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

 ing whether one has a thick or thin one is simply to ignore the funda- 

 mental principles on which correct microscopic images are obtained. 



Fig. 120. Micrometer Calipers (Brown and Sharpe). Pocket Calipers, gradu- 

 ated in inches or millimeters, and well adapted for measuring cover-glasses. 



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. 



§ 226. Cover-Glass Measurers. — For the purpose of measuring 

 cover-glasses there are three very excellent pieces of apparatus. The 



