Prof. Powell: Comparison of Refractive Indices with Theory. 463 
I have satished myself that chloroform and bromoform act like 
iodoform upon triethylphosphine. 
The phosphorus-base acts, even at the common temperature, upon 
tribromide of allyle. The mixture of the two bodies solidifies into a 
erystalline mass, in the examination of which I am engaged. 
The reactions which I have pointed out in this Note have induced 
me to extend my experiments to tetratomic bodies. The chloride of 
carbon, C, Cl,, obtained by the final substitution of chlorine for the 
hydrogen in marsh-gas, appeared to promise accessible results: On 
submitting this body, remarkable for its great indifference under 
ordinary circumstances, to the influence of triethylphosphine, I have 
observed with astonishment a most powerful reaction. Every drop 
of triethylphosphine which is poured into the chloride of carbon, 
hisses like water falling upon red-hot iron. On cooling, the mixture 
solidifies into a mass of white crystals, which will be the subject of a 
special communication. 
December 15.—Sir Benjamin C. Brodie, Bart., Pres., in the Chair. 
The following communication was read :-— 
__ “Comparison of some recently determined Refractive Indices with 
Theory.” By the Rev. Baden Powell, M.A., F.R.S. &e. 
In a series of papers inserted in the Philosophical Transactions 
(1835, 1836, 1837), and afterwards, in a more correct and complete 
form, in my Treatise ‘On the Undulatory Theory applied to the Di- 
spersion of Light’ (1841), I endeavoured to investigate the great 
problem of the explanation of the unequal refrangibility of light on 
the principles of the undulatory theory, as roposed by M. Cauchy 
about 1830, by numerical comparison with the indices observed, 
more especially in cases of the most highly dispersive media then 
examined. 
The general result then arrived at was, that while the theory 
applied perfectly through an extensive range of media of low and 
moderate dispersive power, it did not apply well to those of higher ; 
and to the highest in the scale (which of course formed, the true 
test of the theory) it did not apply within any allowable limits of 
accuracy. Since that time little has been done towards prosecuting 
the subject. 
In the experimental part of the inquiry, about 1849, I had ob- 
served the indices for a few new media*; but these were not high in 
the scale; yet though perhaps thus of little importance, I have now 
thought it as well to go through the calculation for them: the 
results are of the same general character as just described. 
Soon after, finding that my friend, the Rev. T. P. Dale, F.R.A.S., 
was desirous to carry on some researches of this kind, I placed 
at his disposal the apparatus with which I had determined all my 
indicest. 
In 1850 that gentleman communicated to the Royal Astronomical 
Society a short general account of his observations { relative to some 
substances not very high in the scale. 
* See British Association Reports, 1850, Sect. Proce. p. 14. 
+ Described and figured, British Association Reports, 1839, 
t Notices, vol. xi. p. 47. 
