100-0 er. Langley—Determination of Wave-lengths 
mined, are wholly untrustworthy, as will appear more fully 
later. 
Redtenbacher oe the formula 
at bn +5 : 
for expressing the same relation. gUsing the same lines as 
before for determining the unknown constants, we have for the - 
Hilger prism 
1 0-0039220 
—=0°412297—0°00098711— 
- a formula which also satisfies the ope iarens in the visible 
ls ded t 
spectrum, but fails when extende the invisible. The 
curve representing it has a minimum Spoink corresponding to 
n=1°5647 for a value of 2 found from the equation /* =o in 
. eae ae 
the special case of the formula above, where = is positive, 
4=1-430; so that for every value of n greater than 15647, 
there are two real values of 4. This formula therefore is even 
less satisfactory than that of Cauchy. 
riot gives a formula which has been asserted by other — 
investigators* to represent satisfactorily the results of observa: 
tion throughout the whole spectrum, namely: 
Saat o(" a) +e +%(*). 
m four equations like this, using values of n and A cor | 
fecponding to the Fraunhofer lines A, C, F and H, the values 
of the constants were determined+ as follows: e 
a@=0°41028 b=—0-0013495 e—-0-000003379 k= +0°0022329 : 
With the aid of these constants, the wave-lengths correspond: 
ing to given refractive indices were computed, and a curv 
representing the form ula was plotted. This curye, as well E : 
dispersion 
* Mouton, Comptes Rendus, vol. Ixxxix, p. 291 oN vol. a p. 1190. 
is formula has the practical preter ann = leading to eubie 7 
either in 2? or A?, pat solution of which is so tedio as to forbid i u RG 
many places are independently Soest I te teen aided in the p 
lengthy wiille ae pedo by Professor M. B. Goff. 2 
