206 Messrs. J. W. Gifford and W. A. Shenstone. [Jan. 12, 



Interpolation. — In four cases alternative indices obtained by interpola- 

 tion have been given. They are probably more correct for the wave- 

 lengths taken and make a smoother curve. 



The results of the observations are given in the following table of 

 refractive indices. The partial and proportional dispersions of fluorite 

 and vitreous silica are given in Appendix II. 



Befractive Indices of Vitreous Silica. 



1 



Wave-length. Index. Wave-length. 



r 



Index. 



7950 (Rb) ... 

 A' 7682-45 (K a ) ... 

 B' 7065 -59 (He)* . . 

 C 6563-04 (H a ) .. . 

 D 5893-17 (Na) .. . 

 A 5607 -1 (Pb) . . . 

 E 5270-11 (Fe) ... 

 F 4861-49 (H^) ... 

 G-' 4340-66 (H y ) .. . 

 H' 396 L -68 (Al) . .. 



3610 -66 (Cd) . . . 



3302 -85 (Zn) . . . 



3034-21 (Sn) .. . 



Temperature Ref 



1 -453398 

 1 -4538915 

 1 -455180 

 1 -4564147 

 1 -4584772 

 1 -459507 

 1 -4609945 

 1-463165 

 1 -4668500 

 1 -470542 

 1 -475112 

 1 -480610 

 1 -4S6881 



raction coefficie 



274 S -68 (Cd) .. .. 

 2573 -12 „ .... 

 2445 -86 (Ag) .... 

 2312-95 (Cd) .... 

 2265 -13 „ .... 

 2194 -4 „ .... 

 2144-45 „ f.... 



2098-8 (Zn) 



2062 -0 „ 



2024-2 „ + .... 

 1988-1 (Al) .... 

 1933-5 „ §.... 

 1852 -2 „ . . . : 



nt for D for 1° C. -0'000( 



1 -496131 

 1-503707 

 1-51096 

 1 -519373 

 1 -523053 

 1 -529103 

 1 -533898 

 1 -538547 

 1 54271 

 1 -54721 

 1 -551990 

 1 -55998 

 1 -5743 



)0346.|| 



Note. — The number of figures in each index indicate the estimated freedom from 

 errors of observation. The following interpolated indices (see focal curves) are in 

 all probability more correct for the wave-lengths given — 



* 1 -45516. f 1 -53392. + 1 -54728. § 1 "56003. 



|| Communicated on March 3, 1904, in reply to a question by Dr. Watson. 



In conclusion we desire to record the fact that though the cost of 

 the experiments now described was not provided from the Government 

 Grant Fund, yet this work has sprung from earlier work which was 

 so assisted. 



