222 Royal Society. 



istence. The chief objects to be kept in view are, the historical im- 

 portance of the tracts selected for publication, and carefully decy- 

 phering the doubtful contractions that occur in MSS. of the period 

 which this collection is intended to include. The present number 

 being taken as a specimen, we are sure the execution is in good 

 hands. 



XXXI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 152.] 



May 31. — A paper was read, entitled, " Remarks on the Theory 

 of the Dispersion of Light, as connected with Polarization." By the 

 Rev. Baden Powell, M.A., F.R.S., Savilian Professor of Geometr)'' 

 in the University of Oxford. 



The present paper is a sequel to those already presented by the 

 author to the Royal Society, in which he had instituted a compari- 

 son of the observations of the refractive indices for the standard rays 

 of light in various media, with the results calculated from theoretical 

 formulae, deduced from the most improved views of the undulatory 

 hypothesis ; the cases discussed including the greatest range of 

 data as yet furnished by experiment. The comparison exhibited an 

 accordance sufficient to warrant the conclusion that the theory af- 

 fords a very satisfactory approximation, at least, to the expression 

 and explanation of the actual law of nature *. In order, however, to 

 remove any possible discrepancy which may still exist, or hereafter 

 be found to obtain, the author considers that further examination is 

 requisite of the principles on which any extension or modification of 

 the theory might be pursued ; and such is the object of the investi- 

 gation undertaken in the present paper. 



The phenomena of interference, on which the undulatory theory 

 was originally based by Dr. Young, obliged us to adopt some idea 

 of an alternating motion, as well as a motion of translation, in our 

 conception of light ; and this, with all the accessions it has received, 

 especially from the investigation of Fresnel, has, at the present day, 

 been connected by the labours of M. Cauchy and others, with gene- 

 ral dynamical principles, which regulate the propagation of vibratory 

 motions through an elastic medium. From such dynamical prin- 

 ciples there have been deduced certain differential equations of mo- 

 tion, the integration of which gives the well-known expression for a 

 wave, involving the relation between the velocity and the wave- 

 length which explains the dispersion. The direct and complete in- 

 tegration of these forms, effected by M. Cauchy f, and simplified by 



* A notice of Prof. Powell's last 'paper on the subject was given in 

 vol. xii. p. 367.; where also will be found references to abstracts of the 

 three former. — Edit. 



t Prof. Powell's " Abstract of the Essential Principles of M. Cauchy's 

 View of the Undulatory Theory," appeared in Lond. and Edinb. Phil. Mag. 

 vol. vi. p. 16. etseq. 



