W. Gibbs—Miscellaneous Optical. Notices. 45 
Arr. VII.—WMiscellaneous Optical Notices; by WoLcorr Grsss, 
M.D., Rumford Professor in Harvard Universit ity. 
oa. 
On the measurement of wave ans ie oy means of indices of re- 
rnetiens 
In a brief notice t NEE to the British Association 
for the Advancement of Science at its meeting in 1849, Prof. 
Stokes has given a method for measuring wave lengths, which 
depends upon the fact that, in substances of medium refractive 
power, the increment of the index of refraction in passing from 
one point of the spectrum to another is nearly proportional to 
the increment of the square of the reciprocal of the wave 
length. The author showed that even when the intervals were 
bee much longer than necessary, the error in the wave length 
as usually only in the eighth p ace of decimals. At the date 
of "the publication of this notice the subject of wave lengths 
possessed but little interest. The recent development of the 
spectral analysis of light has rede a new impulse to this branch 
= eee. and has _— Fae omen the construction of a 
p of the en lar spectrum. This has been most 
accom Tished by Angstrém,} but an attentive study 
of pie work, as well as of the elaborate researches of Van der 
Willigen§ and Ditscheiner A will show that new measurements 
e far from superfluous. The imperfections even of the 
Ft give first a pre of the method of Stokes in a oe 
inal form, and meiped a simplification of that method org 
will also have its 
peg ae a: fe 
If Cauchy’s pate for dispersion, n=a+ +73; 
* Read before the National ee es Seen se 
rt of the British Adsouation for the Advancon Sak eh faioaen for 1240. 
Notices and abstracts, p. 10. 
rches sur le Spectre Solaire. 1869 
Archives du Musée Teyler, vol. i, p. 1 
Sitzungsberichte der k. k. Akad. der Wissenschaften Bd. 1, 1864. 
“{ This Journal, xlvii, , 1869. 
¥* Mé roe ess | étalliques, 1868. 
