342 F. J. CHESHIRE ON FOCOMETRY AND APERTOMETRY. 



and must be so. There is, however, a large class of micro- 

 scopists, whose objectives are practically confined to dry ones, 

 Avho would, I venture to think, be glad to be able to compare and 

 determine the N.A's of their objectives, and to satisfy themselves 

 that they are what they profess to be. To that class my paper is 

 addressed. 



My thanks are due to Mr. Curties and Mr. Angus for their 

 courtesy in placing at my disposal apparatus necessary for my 

 experiments. To a late pupil of mine, Mr. Hailes, I am under 

 obligation for the careful drawing of the various diagrams. 



Joum. Quekett Microscopical Club, Ser. 2, Vol. VJIJ., No. 51, November 1902. 



