SPECULUM METAL RULINGS 35 



cate instruments on speculum metal up to 

 14,000 lines, and even more, per inch, but so 

 far Mr. Grayson has far surpassed all other 

 records in microscopic rulings. His plates 

 contain 120,000 lines per inch. 



Mr. E. M. Nelson, one of the greatest 

 authorities on microscopic research, tells us 

 his experience in resolving those lines ; it 

 need hardly be said that such fine work 

 requires uncommonly good microscopic powers 

 and excellent illumination for their resolution. 



Mr. Nelson found that he could resolve 

 strongly the 120,000th band with a th inch 

 oil immersion lens ; he also resolved the same 

 lines with a cheap -f^ih inch oil immersion. 



The 90,000th band he resolved with the |th 

 inch, and also with a Powell and Lealand's 

 best | inch ; and the 60,000th band with a 

 Zeiss J inch. (Royal Micros. Journal, 1904). 



The rulings are mounted in realgar, and are 

 more difficult to resolve than diatoms of equal 

 fineness. 



This being Mr. E. M. Nelson's experience, 

 the statement made in the latest works on 



