THE 
LONDON, EDINBURGH anv DUBLIN 
PHILOSOPHICAL MAGAZINE 
AND 
JOURNAL OF SCIENCE. 
[FOURTH SERIES.] 
MARCH 1860. 
XXII. On certain Laws of Chromatic Dispersion. 
By Muneo Ponton, F.R.S.E.* 
time attempts hitherto made to determine the laws of chro- 
matic dispersion have generally proceeded on the principle 
of assuming a certain law, and endeavouring, on its basis, to con- 
struct a universal formula, which shall render it possible, from 
having given the refractive indices of two or three of the fixed 
lines of the spectrum for any medium, to find those of the 
remainder. These attempts are known to have been attended 
with but imperfect success. 
It has accordingly been deemed expedient to adopt a different 
course,—to assume the indices of al/ the seven fixed lines for 
every refracting medium that has been examined, to be, as deter- 
mined by observation, nearly correct ; to analyse and compare 
these indices with a view to discover the hidden law or laws 
which they involve ; and, the general nature of these laws having 
been thus brought to light, then to make on the observed indices 
such corrections as may be required to bring them into strict 
agreement with the laws so ascertained. Thus, without assu- 
ming any one or more of the observed indices to be absolutely 
accurate, as in the former method of proceeding, it may be found 
possible so to correct each index as to obtain results harmo- 
nious in themselves, and all agreeing with determinate general 
laws. The following is a brief outline of the results to which 
this method of research has led. 
The most general forraula under which the law of chromatic 
dispersion can be expressed is the following. Let U represent 
* Communicated by the Author, having been read before the British 
Association at Aberdeen, September 1859. 
Phil, Mag. 8, 4, Vol. 19. No. 126. March 1860. N 
