164 



SLIDES AND COVER-GLASSES 



VII 



In studying the preparation one would not try to use objectives of a shorter work- 

 ing distance than the thickness of the cover (61); (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 thickness of cover may be selected corresponding to that for which 

 the objective was corrected (see table, \ 29). 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 

 (\ 102, Fig. 133.) 



In the so called No. i cover-glasses of the dealers in microscopical supplies, 

 the writer has found covers varying from o. 10 mm. to 0.35 mm. To use cover- 

 glasses of so wide a variation in thickness without knowing whether one has a 

 thick or thin one is simply to ignore the fundamental principles by which correct 

 microscopic images are obtained. 



32nds. 

 1 .0312 

 3 .0.937 

 5 .1562 

 7 .2187 

 9 .2812 

 11.3437 

 13.4062 

 15.4687 



FIG. 132. 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 covered with 

 a cover-glass whose thickness is known, and that this thickness should be indicated 

 in some way on the preparation. 



| 241. Cover-Glass Measurers, Testers or Gauges. For the purpose of 

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

 micrometer calipers (Fig. 132) used chiefly in the mechanic arts, are convenient 

 and from their size easily carried in the pocket. The two cover-glass measurers 

 specially designed for the purpose are shown in Figs. 133-134. With either of 

 these the covers may be more rapidly measured than with the calipers. 



With all of these measures or gauges one should be certain that the index 

 stands at zero when at rest. If the index does not stand at zero it should be 

 adjusted to that point, otherwise the readings will not be correct. 



As the covers are measured the different thicknesses should be put into 

 different boxes and properly labeled. Unless one is striving for the most accurate 

 possible results, cover-glasses not varying more than 0.06 mm. may be put in the 

 same box. For example, if one takes 0.15 mm. as a standard, covers varying 0.03 

 mm. on each side may be put into the same box. In this case the box would con- 

 tain covers of 0.12, 0.13, 0.14, 0.15, 0.16, 0.17 and o.iS mm. 



