250 



SLIDES AND COVER-GLASSES 



[C//. JX 



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 corrected (see table, 2 33). Furthermore, if there is 

 a variation from the standard, one may remedy it, in part at least, by lengthen- 

 ing the tube if the cover is thinner, and shortening it if the cover is thicker 

 than the standard ( 113). 



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

 supplies, the writer has foxmd 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. 



It is then strongly recommended that every preparation shall be covered 

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

 cated in some way on the preparation. 



2 338. Cover-Glass Measures, Testers or Gauges. For the purpose of 

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

 The micrometer calipers (Fig. 195) used chiefly in the mechanic arts, are con- 

 venient and from their size are easily carried in the pocket. The cover-glass 

 measurer specially designed for the purpose is shown in Fig. 196 by which 

 covers may be more rapidly measured than with the calipers. 



FIG. 196. Zeiss' Cover-Glass 

 JMeasurer. With this the 

 knife edge jaics are opened by 

 means of a lever and the cover 

 inserted. The thickness may 

 then be read off on the face as 

 the pointer indicates the thick- 

 ness in hundredths millimeter 

 iu the outer circle and in thous- 

 andths inch on the inner circle- 



With 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 glass boxes and properly labeled. Unless one is striving for the most 

 accurate possible results, cover-glasses not varying more than 0.06 mm. maybe 

 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 contain covers of o. 12, 0.13, 0.14, 0.15, 0.16, o 17 and o.iS mm. 



\ 339. Bichromate Cleaning Mixture for Glass. The cleaning mixture 

 used for cleaning slides and cover-glasses is that commonly used in chemical 

 laboratories : (Dr. G. C. Caldwell's Laboratory Guide in Chemistry). 



