Prof. Powell on the Dispersion of Light, 367 



stances where the amount of the circulating force is greater, than in 

 the contrary cases. 



3rd. That the amount of force put into circulation from a large 

 surface of zinc towards a central ball of copper, is, as in former in- 

 stances of similar combinations, about one half of that from the re- 

 verse arrangement. 



4th. That a ball of zinc, exposing a surface of 3-14 square inches, 

 placed over the centre of a plate of copper, exposing on its two sides 

 a surface of 28 square inches, sustains an action of nearly the same 

 amount as a plate of zinc, of the same dimensions as the copper, 

 placed at the same distance. 



In conclusion, the author remarks, that the principal circumstance 

 which limits the power of an active point within a conducting sphere, 

 in any given electrolyte, is the resistance of that electrolyte, which 

 increases in a certain ratio to its depth or thickness ; and this thick- 

 ness may virtually be considered the same wherever the included 

 point may be placed, but increases with the diameter of the sphere. 

 In an insulated hemisphere, however, the approximation of the active 

 point to the lower surface virtually decreases the thickness of the 

 electrolyte, and consequently the force increases. In this respect, 

 the action of a point upon a plate may be considered the same as 

 upon an indefinitely large hemisphere, towards which, as the point 

 approaches, the force increases. 



Feb. 8. — A paper was read, entitled, " Researches towards 

 estabhshing a Theory of the Dispersion of Light", No. IV.* By the 

 Rev. Baden Powell, M.A., F.R.S,, Savilian Professor of Geometry 

 in the University of Oxford, 



In his former communications to the Royal Society the author 

 had instituted a comparison of the results of observation and of 

 theory with regard to the dispersion of light, in the instances of the 

 respective indices for the standard rays in fifteen different cases of 

 transparent media ; and had found a suflliciently close agreement in 

 the cases which gave the lower numbers ; but there yet appeared to 

 be an increasing discrepancy as an advance was made towards the 

 higher. The theoretical formula employed in this investigation was 

 one derived from the undulatory hypothesis, by a process involving 

 some limitations, which rendered it only approximative. By pur- 

 suing the calculations to a greater degree of development, or by 

 adopting methods of a more precise character, such as those of M. 

 Cauchy and of Mr. Kelland, the author was led to hope that a more 

 close coincidence might be obtained. The formulae of M. Cauchy, 

 however, involved calculations of so elaborate and overwhelming a 

 character, that he was induced to make trial of the method of Mr. 

 Kelland, applying it, in the first instance, to the case of the most 

 highly dispersive substance, namely, oil of Cassia, in which the 

 greatest discrepancy had before appeared. 



The object of the present communication is to state the results 

 obtained, together with the necessary data employed in the calcu- 



* Abstracts of Prof. Powell's former papers on this subject have been given 

 in vol. Ti, p. 374 } vol. viii. p. 413, • vol. x. p, 221 : see also vol. xi. p. 477. 



