842 



Transactions of the Society. 



XLII. — AiJerture Measurements of Immersion Objectives ex 

 as " Numerical Aperture." 



By John Mayall, jun., F.E.M.S. 



CBead 12th November, 1879.) 



As it is probable that apertures will be expressed, in future scientific 

 discussions, by the nomenclature suggested by Professor E. Abbe, 

 i. e. '* Numerical Aperture," the following list of actual measurements 

 may be interesting to microscopists : — 



Zeiss's homogeneous immersion (Abbe's formula,) 



1879} J 



Tolles"s oil immersion (1879) 



„ water immersion (1876) 

 Zeiss's homogeneous immersion (1878) 

 Tolles's water immersion (1877) 

 Zeiss's homogeneous immersion (1878) 



Tolles's water immersion (1875) 



(187(3) .. .. 

 Powell and Lealand's oil immersion (1879) 

 Tolles's water „ (1874) 



Powell and Lealand's „ „ new! , 



formula (1875) / * 



Tolles's „ „ (1874) be-1 , 



longing to Mr. Crisp J ' 



Powell and Lealand's „ „ new^ , 



formula (1875) j '^ 



,. ., (1874) .. J^ 



„ „ „ „ new"! J 



formula (1876) j '^ 



Tolles's „ „ (1876) .. J, 



Powell and Lealand's 



formula (1876) .. 

 Prazmowski's 



Zeiss's 



Nobert's 

 Hartnack'i 



Prazmowski's 

 Giindlach's 



(1874) No. 8 

 (1875) .. 



(1867) .. J^ 

 „ No. 12 = J^ 

 ,, No. 9 = ,v 



(1874) .. . 



15 + 



15 + 



15 



15 



12 

 10 



* I am indebted to Professor E. Keith (of U.S.A.) for this measurement. He 

 measured the balsam angle to be 115° (the highest aperture attained at that date), 

 which would be about 1'28 of "numerical aperture." The other measurements 

 were made by myself, and several were verified by Professor E. Abbe. 



