68 



sufficient to seriously affect and the high powers 

 totally obliterate the definition which under normal 

 conditions it may possess. The microscopist is there- 

 fore not obtaining such results as his objectives 

 ought to enable him to obtain, and the efforts of the 

 conscientious optician to provide classified objectives 

 of reliability and similar performance is almost 

 entirely nullified. 



"The system which I have devised to aid in overcom- 

 ing these difficulties depends in the first instance up- 

 on a micrometer for measuring the thickness of cover 

 glass. See Fig. 18. 



"In objectives provided with cover correction the 

 graduation is so arranged as to read to y i- mm. No 

 matter what the power of objective, the number gives 

 proper correction for a thickness corresponding 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, consequently, the best 

 results. 



"All the other scales give the correct tube length 

 in inches and millimeters for covers corresponding 

 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 approxi- 

 mately if not entirely. At any rate, the micrometer 

 will detect the requirements before using the covers, 



