and since resolving power depends primarily upon the 

 proper correction of the two aberrations, it will be 

 entirely lost if there is much variation from the normal 

 thickness. 



It must not be forgotten that in microscopical work 

 we are dealing with minute things and this applies 

 especially to the cover glass. By studying the table 

 which follows it will be noted that there is quite a 

 difference in thickness used by the various makers, and 

 that the mean thickness is 0.18 mm. or 0.007 inch. 



The deviation from standard thickness affects the 

 distinctness of the image according to its structure and 

 in proportion to the increase in power. In the low 

 powers there is no noticeable influence, but with the 

 1-5, 1-6, and 1-8 it is so marked that with objects of 

 fine structure. a deviation of 0.05 mm. either thicker 

 or thinner than the standard is sufficient to totally 

 obliterate fine structure and have the outlines and 

 coarse lines only apparent. Slighter variations affect 

 the image proportionately. 



It is surprising to see how little attention is paid to 

 this subject in the large majority of the standard works 

 on the microscope. Almost all books give carefully 

 prepared illustrations and descriptions showing the 

 effect on the course of light of the interposition of the 

 cover glass, and after giving conclusive evidence of its 

 disturbing influence, still, in a general way, say it is of 

 little moment. 



9 



