Rev. Prof. Powell on the Dispersion of Light. '2'2S 



Mr. Tovey* and M. Kellandf, involves certain conditions; namely, 

 the evanescence of certain terms, the interpretation of which implies 

 peculiar views of the constitution of the ether, Mr. Tovey shows 

 that without these conditions, a certain form of the wave-function 

 is a particular solution of the equations ; and this form is precisely 

 that expressing elliptically polarized light. If the absence of the 

 condition in question be essential to the case of elliptically and cir- 

 cularly polarized light, it follows that all the preceding investiga- 

 tions, which depend on the fulfilment of those conditions, are ap- 

 plicable only to unpolarized and plane-polarized light, and conse- 

 quently the general integration is limited in a most material part of 

 its application ; a defect which is only remedied by the supplement- 

 ary investigation of Mr. Tovey, in which, for this case, a particular 

 solution is assigned. It seemed, then, necessary to show explicitly 

 that the non-fulfilment of the conditions, that is, the non-evanes- 

 cence of the terms in question, is essential for elliptically polarized 

 light, as their evanescence is for common light, and thus to exhibit 

 distinctly the relation between the cases of elliptically polarized, of 

 plane-polarized, and unpolarized light ; and, again, to remove, if 

 possible, the obscurity and discrepancy of opinion in which the phy- 

 sical interpretation of those conditions, with regard to the supposed 

 constitution of the ethereal medium, appeared to be involved. 



The author then enters upon the analytical investigation of the 

 subject, and in conclusion remarks that when light is elliptically or 

 circularly polarized, that is, when any one of the two component 

 vibrations is retarded behind the other, then, in the differential equa- 

 tions of motion, the opposite terms do not destroy each other in the 

 summation, which they can only do in general by supposing a great 

 number taken into account ; that is, the number of terms is limited, 

 or the sphere of the influence of the force by which the vibrations 

 are propagated is small. When light is plane-polarized,- or unpo- 

 larized, that is, when there is no retardation, or the phases of the 

 component vibrations are simultaneous, then the opposite sums de- 

 stroy each other; that is, the number of terms involved is greater, 

 or the sphere of the influence of the force greater. Since both kinds 

 of light can be propagated indifferently through ordinary media, it 

 follows that the sphere of influence of the force, or number of mole- 

 cules taken into account, does not here depend on the arrangement 

 of the molecules of ether in the medium, but on the retardation of 

 one of the vibrations behind the other, or the absence of it, origin* 

 ally impressed on the ray in the respective cases. 



A paper was also read, entitled, "An Experimental Inquiry into the 

 influence of Nitrogen on the Growth of Plants." By Robert Rigg, 

 Esq. Communicated by the Rev. J. B. Reade, M.A., F.R.S., &c. 



The author, after briefly alluding to a former paper laid before the 

 Royal Society, describing the chemical changes which occur during 



* Mr. Tovey's investigations on this subject have appeared, exclusively, 

 we believe, in this Journal : see Lond. and Edinb. Phil. Mag., vol. viii. p. 7-j 

 270, 500.; vol. ix. p. 420.; xi. 524.; and xii. p. 10, 259. 



f Prof. Kelland's development of his views will be foundin vol. x. p. 336. 



