ON MOUNTING. 133 



circles, squares, oblongs and ovals and of any 

 required size ; circular or oval covers will be found 

 the most convenient to use, and the neatest in 

 appearance; circular covers can be "ringed" by 

 the aid of the turn table with any of the various 

 protective cements. Cover glasses are supplied of 

 three thicknesses, known respectively as No. 1, 

 No. 2, and No. 3. No. 3 covers are to be avoided 

 as too thick. No. 2 will be found quite thin enough 

 for all ordinary purposes. No. 1 must be used for 

 all preparations intended for the higher powers. 



An ounce of No. 1 covers should have a fair pro- 

 portion of *003 inch glasses in it, still more of "004 

 than *005, advancing in proportion from "003 to *006. 

 Should an ounce of No. 1 covers contain more than 

 a dozen, or at most a score, over *006, it ought to 

 be rejected. As the objectives now used are oil 

 immersion, or ought to be, for powers higher than 

 an eighth the thickness of the cover may be of less 

 importance. For instance, Messrs. Powell and 

 Lealand's -fs oil immersion lens works through a 

 cover of '006 inch perfectly. Almost anything in 

 histological work can be seen with an objective of 

 a e inch power : only exceptionally do we require 

 so high a power as the A inch. We ought, how- 

 ever, to use no cover thicker than a '006 inch, if 

 possible. There are numerous cover-glass measures. 

 Messrs. Ross make an excellent one. 



Mounting in Canada Balsam. 



Many years ago the author discovered the 

 following method of using Canada balsam, and 

 having made no secret of it, it is generally known 



