143 



limits of different numbers. The variation is 

 about as follows : 



No. 1, Y^Q to -g-^Q inch, or O.i6 to 0.13 mm. thick ; 

 No. 2, yfoj. to yfo inch, or 0.25 to 0.16 mm. thick ; 

 No. 3 -L. to Jf- inch, or 0.50 to 0.26 mm. thick. 



According to the prices of cover glasses, when 

 purchased by weight, the No. 1 gives the greatest 

 number and No. 3 the least. It may for this 

 reason be thought that the purchase of No. 1 is 

 most advantageous, but it must be considered that 

 there is a greater amount of breakage by cleaning, 

 as they are very thin and sensitive. Considered 

 from the optical standpoint, the thinner of No. 2 

 and the thicker of No. 1 should be used, as these 

 come within limits which are adopted by opticians 

 as standard and to which medium and high power 

 objectives are corrected. Test objects which are 

 prepared to test the reserving power of objectives 

 and consist of diatoms, are generally covered with 

 these thicknesses. 



An excellent means of determining the thick- 

 ness of cover glass, as well as of studying spherical 

 and chromatic aberration, is the Abbe test-plate. 

 This consists of a series of cover glasses ranging 

 in thickness from 0.09 mm. to 0.24 mm., silvered 

 on the under surface, with lines cut through the 

 film, and cemented to a slide, each cover being 

 marked with its thickness. 



