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II. Researches towards establishing a Theory of the Dispersion of Light. No. III. 
By the Rev. Baden Powell, M.A. F.R.S., Savilian Professor of Geometry in the 
University of Oxford. 
Received October 20, 1836 — Read January 19, 1837. 
Introductory Remarks. 
In two former portions of researches on the subject of dispersion *, I have discussed 
all the observed refractive indices for definite rays, in different media, which had 
come to my knowledge ; consisting of those for ten media determined by Fraunhofer, 
and those in ten other cases by M. Rudberg, comparing them with the calculated 
results of the theory of M. Cauchy ; and the agreement is sufficiently close. In 
those papers, and elsewhere, I have remarked the importance of extending the in- 
quiry, especially to media of higher dispersive power ; in which cases (as appears 
from the nature of the formula) the theory would be put to a more precise test. 
In the former instances the work of determining the indices was done to my hands, 
and I could proceed to the theoretical computations with the most perfect confidence 
in the accuracy of experimental data, furnished from the labours of observers so well 
known for precision and skill, and obtained, too, before the formula of theory had 
been deduced. 
In any comparison of theory with experiment, it is, in all points of view, far more 
satisfactory that such comparison should be made with the observations of others 
rather than those of the theoretical computer himself. In the present instance, how- 
ever, this desirable condition has not been fulfilled. Though the importance of ob- 
taining a series of indices for the standard rays in different media had been long since 
pointed out and acknowledged by the most eminent philosophers, yet no observer was 
found to undertake the task of carrying on the work which Fraunhofer and Rud- 
berg had so successfully begun. I was thus left to make an attempt myself to supply 
the deficiency ; and my observations were communicated, and printed copies distri- 
buted, to the Physical section of the British Association at the Bristol meeting in 
August 1836T- 
From the remarks prefixed to those results, the scientific reader will, I trust, be 
sufficiently enabled to judge of the nature and degree of accuracy of the observations. 
* Philosophical Transactions, 1835, Part I. ; and 1836, Part I. 
t This tract now forms one of the series of memoirs published by the Oxford Ashmolean Society. 
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