63 



by merely dropping a cover-glass upon it all these 

 objections are overcome. 



The above are merely practical considerations, but 

 there are others of a theoretical nature and of as 

 much importance. After a high power objective has 

 been corrected to a certain thickness of cover, any 

 variation, not necessary considerable, has an injurious 

 effect upon the spherical corrections, and consequently 

 upon the resolving power. It is manifest that the 

 quality of the latter will decrease as the variation 

 increases, and when it reaches a point where no cover 

 is used, it may be so considerable as to destroy an 

 accurate perception of what is sought. 



In this connection it is considered important to 

 state what thickness of cover-glass it is best to use. 

 As is probably well known, there are three grades, 

 which are designated as No. 1, No. 2 and No. 3. 

 Although they are classified, there is a variation with- 

 in the limits of different numbers The variation is 

 about as follows : No 1, T ^ to -^ inch thick ; No. 

 2, yffl- to T ^ inch thick ; No. 3, -^ to -^ inch thick ; 

 According to the prices of cover-glasses, when pur- 

 chased by weight, the No. 1 give the greatest number 

 and No. 3 the least. It may for this reason be 

 thought that the purchase of No. 2 is most advan- 

 tageous, but it must be considered that there is a 

 greater proportion of breakage by cleaning, as they 

 are very thin and sensitive. Considered only from a 

 optical standpoint, No. 2 should generally be used, as 

 the medium and high power objections are adjusted 



