326 



March 12, 1835. 



The Rev. PHILIP JENNINGS, D.D., Vice-President, in the Chair. 



Continuation of a former paper " On the twenty-five feet Zenith 

 Telescope, lately erected at the Royal Observatory ; " by John Pond, 

 Esq., F.R.S., Astronomer Royal. 



For determining the place of any star passing the meridian near the 

 zenith, at the Royal Observatory at Greenwich, three different me- 

 thods may be employed : first, by means of the mural circles j se- 

 condly, by the zenith telescope, used alternately east and west ; and 

 lastly, by means of a small subsidiary angle, as described by the au- 

 thor in a former paper. The details of computations made according 

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

 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 

 in 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 deducedfrom 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 phenomena of the di- 

 spersion of light. 



March 19, 1835. 



Sir JOHN RENNIE, Knt., Vice-President, in the Chair. 



A paper was read, entitled, " Some Account of the Eruption of 

 Vesuvius, which occurred in the month of August, 183 4, extracted 

 from the manuscript notes of the Cavaliere Monticelli, Foreign As- 

 sociate of the Geological Society, and from other sources ; together 

 with a Statement of the Products of the Eruption, and of the Con- 



