The Microscope. 275 



Carl Zeiss makes one especially for the niicroscopists, but either 

 of the three manufactured in this country, especially that of 

 Bausch and Lomb, will answer equally well. It is not only con. 

 venient but important to have covers of different thicknesses 

 sorted out, so that almost any kind may be had at a moment's 

 notice, and the micrometer gauge is slightly more accurate in its 

 results than the fine adjustment screw, with which the thickness 

 may also be estimated. An experiment made while writing 

 gives the thickness of a cover as measured by a gauge to be 

 one two-hundreth inch, while the fine adjustment screw gives 

 0.0045, or about 1.223 inch. 



To use the former, place the cover between the jaws of the 

 instrument, close them and read the thickness in hundredths and 

 thousandths of an inch. To use the fine adjustment screw for 

 the purpose, if the milled head be graduated and the value of 

 the spaces known, it is only necessary to count the number of 

 divisions emploj'ed in focussing from one surface of the glass to 

 the other. A few particles of dust will answer as objects on 

 which to focus, or a minute drop of ink on each side of the 

 cover, and so close as to be together in the field of a high power 

 ■objective, but not close e iOugh to overtop each other. In the 

 experiment referred to, the number of the divisions on the 

 milled head used in focussing from the lower to the upper sur- 

 face of the cover, where four and one-half, and as each division 

 corresponds to a movement of the body tube of one one-thou- 

 sandth inch, the cover was four and one-half thousandths, or 

 1-223 inch thick. 



The most desirable as well as the cheapest and the best 

 micrometer gauge now in the market is made by Messrs. Bausch 

 and Lomb. This was described by its inventor, Mr. Edward 

 Bausch, in The Microscope for October, 1890. To that paper 

 the reader is referred. 



