CH. X] MEASURING SLIDES AND COVERS 319 



ing the preparation one would not try to use objectives of a shorter 

 working distance than the thickness of the cover ( 76) ; (b) hi using 

 adjustable objectives with the collar graduated for different thick- 

 nesses of cover, the collar can 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 cor- 

 rected ( 460) . Furthermore, if there is a variation from the stand- 

 ard, 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 (462). 



Among the so-called No. i cover-glasses of the dealers in micro- 

 scopical supplies, the writer has found covers varying from o.io 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. 



From information supplied by Mr. Edward Pennock the thickness 

 of various cover-glasses should be within the following limits: 



No. i cover-glasses. . . .0.12 to 0.18 mm. 



No. 2 0.18 to 0.25 mm. 



No. 3 0.25 to 0.50 mm. 



No. o o.io mm. slightly more or less. 



In general cover-glasses thinner than the minimum (0.12 mm.) of 

 No. i, actual measurement, will, as stated above, usually show 

 a much wider variation. 



It is then strongly recommended that every preparation shall be 

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

 thickness be indicated in some way on the preparation. 



496. Cover-glass measures, testers, or gauges. For the pur- 

 pose of measuring cover-glasses there are two very excellent pieces of 

 apparatus. The micrometer calipers (fig. 188), used chiefly in the 

 mechanic arts, are convenient and from their size are easily carried in 

 the pocket. The cover-glass measurer specially designed for the 

 purpose is shown in fig. 189, by which covers may be more rapidly 

 measured than with the calipers. 



