374 Royal Society. 



to each of these three methods are contained in the present paper j 

 from which it appears that they all give results nearly identical ; and 

 that, when the observations with the two circles are made with suf- 

 ficient care, the greatest error to be apprehended does not exceed 

 the quarter of a second. 



*' Remarks towards establishing a Theory of the Dispersion of 

 Light." By the Rev. Baden Powell, M.A., F.R.S., Savilian Professor 

 of Geometry in the University of Oxford. 



In an abstract of M. Cauchy's Theory of Undulations, published 

 hi the London and Edinburgh Journal of Science*, the author of the 

 present paper deduced a formula expressing precisely the relation 

 between the length of a wave and the velocity of its propagation ; and 

 showed that this last quantity is, in fact, the same as the reciprocal 

 of the refractive index. The author here examines, by means of this 

 formula, the relation between the index of refraction and the 

 length of the period, or wave, for each definite ray, throughout the 

 whole series of numerical results which we at present possess ; and 

 the conclusion to which he arrives from this comparison, for all the 

 substances examined by Frauenhofer, viz. for four kinds of flint glass, 

 three of crown glass, water, solution of potash, and oil of turpentine, 

 is that the refractive indices observed for each of the seven definite 

 rays are related to the length of waves of the same rays, as nearly as 

 possible according to the formula above deduced from Cauchy's theory. 

 For all the media as yet accurately examined, therefore, the theory of 

 undulations, as modified by that distinguished analyst, supplies at 

 once both the law and the explanation of the phaenomena of the di- 

 spersion of light. 



March 19. — A paper was read, entitled, " Some Account of the 

 Eruption of Vesuvius, which occurred in the month of August, 1834, 

 extracted from the manuscript notes of the Cavaliere Monticelli, 

 Foreign Associate of the Geological Society, and from other sources ; 

 together with a Statement of the Products of the Eruption, and of 

 the Condition of the Volcano subsequently to it." By Charles Dau- 

 beny, F.R.S., F.G.S., and Professor of Chemistry in the University 

 of Oxford. 



It appears, from the information collected by the autlior, that for a 

 considerable time previously to the late eruption of Vesuvius, stones 

 and scoriae had been thrown up from the crater, and had accumulated 

 into two conical masses, the largest of which was more than two hun- 

 dred feet in height. On the night of the 24th of August last, after 

 the flow of considerable currents of lava, a violent concussion took 

 place, followed by the disappearance of both these conical hillocks, 

 which, in the course of a single night, were apparently swallowed up 

 within the cavities of the mountain. Fresh currents of lava continued 

 to flow for several days subsequently, destroying about 180 houses, 

 spreading devastation over a large tract of country, and destroying all 

 the fish in the neighbouring ponds and lakes. After the 29th of August, 

 no further signs of internal commotion were manifested, with the ex- 

 ception of the disengagement of aqueous and aeriform vapours from the 

 * See our present volume, p. \Qct seq. 



