144 



SLIDES AND COVER-GLASSES. 



[CTf. VII. 



micrometer calipers, used chiefly in the mechanic arts, is convenient, and 

 from its size easily carried in the pocket. The two cover-glass meas- 

 urers, specially designed for the purpose, are shown in Figs. 120-122. 

 With either of these the covers ma)' be more rapidly measured than with 

 the calipers. 



With all of these measurers 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. 



Fig. 121. Cover-Class Measurer (Edward Bausch). 



The cover glass is placed in the notch between the hvo screws, and the drum is 

 turned by the milled head at the right till the cover is in contact with the screws. 

 The thickness is then indicated by the knife edge on the drum, and may be read off 

 directly in s } Ui th mm. or u ^ ru - t th inch. In other columns is given the proper tube- 

 length for various unadjustable objectives ( }, ±, \, and fa in. ) made by the Bausch 

 and Lamb Optical Company. 



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 



