1904.] The Optical Properties of Vitreous Silica. 



207 



Appendix I. 



Table giving the Focal Lengths in Metres of a Compound Lens 

 of Fluorite and Vitreous Silica, achromatised for Wave-lengths 7950 

 and 1852. 



Radii — E = 0-38733, S = 0-20351, E' = S, S' = oo. E, S, E', S' 

 refer to the surfaces of the two lenses. 



Wave-length. 



Focal length. 



Wave-length. 



Focal length.. 



7950 



1 -ooooo 



2749 



-99250 



7682 A' 



1 -oooio 



2573 



-99143 



7066 B' 



1 -00045 



2446 



99054 



6563 C 



1 -00070 



2313 



99055 



5893 D 



1 -000S6 



2265 



-99078 



5607 



1 -00059 



2194 



-99120 



5270 E 



1 -00017 



2144 



-99151 



4861 F 



-99983 



2099 



0-99174 



4341 G' 



-99874 



2062 



-99205 



3962 IF 



-99743 



2024 



0-99258 



3611 



-99653 



1988 



-99360 



3303 



-99558 



1933 



-99490 



3034 



-99409 



1852 



1 -ooooo 



