216 HOW TO SEE WITH THE MICROSCOPE. 



objectives under which he expects to show them, and 

 let him, too, early learn to discriminate as to the dif- 

 ferences in thickness of covering glass. 



A handy instrument for measuring the thickness of 

 thin glass is a little pocket micrometer gauge manufac- 

 tured by the Brown & Sharp Company, of Providence, 

 R. I. Although not expressly made for the purpose, it is 

 as convenient and accurate as any other we have met, 

 while its cost is very moderate.* We make it a plan that 

 when a quantity of new covers are first in hand to pass 

 them over to the pupil, to be by him measured with the 

 instrument aforesaid, and assorted according to their 

 thickness, the various styles being kept in little boxes 

 by themselves, and duly labeled. In this way the stu- 

 dent acquires the needful practice, and the requirements 

 of the laboratory are met at the same time. If so be 

 that no instrument of the kind is at hand, or that the 

 observer feels that he cannot afford the necessary out- 

 lay, the usual fine adjustment of the stand can be made 

 to do tolerably effectual duty, thus : Obtain a cover, of 

 which the thickness is known, and with a fine pen dipped 

 in thick India ink, make a mark on one surface near the 

 centre; turn the cover over and make another similar 

 mark nearly, but not exactly opposite the first . Now 

 place the same under the microscope, focus on the mark 

 on the under side -of the cover, and then, by the fine 

 wheel of the fine adjustment, focus again on the mark 

 on the upper surface, noting the revolutions or parts 



* As we have before remarked, the fine adjustment of the Acme can be 

 used as a micrometer. 



