418 . Transactions of the Society. 



power necessary, when there is no "lag," to resolve 50,000 lines per 

 inch ? Here 437 multiplied by 50 = 21,850, dividing by 100 the 

 answer 218 is obtained 



The N.A. necessary to resolve any piven number of lines to the 

 inch with a large solid axial cone and Giflbrd's screen, may be found 

 by dividing t or the number of thousands of lines to the inch by 80. 

 Example : — What is the N.A. necessary to resolve 50,000 lines per 



inch ? Here t = 50, and — = • 625, the N.A. required.* 



80 H 



The test-plate was a 12-band plate, ruled to 60,000 lines per inch, 



by Mr. H. J. Grayson, of Melbourne, mounted in realgar of refractive 



index 2 • 5. 



* See tin's Journal, 1893, p. 17. 



