102 



" In objectives provided with cover correction 

 the graduation is so arranged as to read to T ^ ^ mm. 

 No matter what the power of objective, the num- 

 ber gives proper correction for a thickness corres- 

 ponding to it. Thus, with a cover glass of 0.20 mm. 

 the collar of such an objective need merely to be 

 set at 20 to give the proper correction and, conse- 

 quently, the best results. 



All the other scales give the correct tube 

 length in inches and millimeters for covers corres- 

 ponding to them, and in this manner offer a ready 

 and definite means of correction. The tube lengths 

 required for the thinnest and thickest covers are so 

 extreme that probably no convenient means for 

 obtaining them can be practically arranged, but 

 they can be so, approximately if not entirely. At 

 any rate, the micrometer will detect the require- 

 ments before using the covers, and those deviating 

 considerably from the normal can be used on 

 objects for use with low powers only, in which 

 case the effect will not be very appreciable. 



" Iri this system I do not overlook the fact that 

 variation in tube length involves a variation in 

 magnifying power ; but, except in cases where 

 micrometers are used, I consider this of secondary 

 importance, as it always is in comparison to results 

 obtained in resolving power. 



