TRANSACTIONS OF THE SECTIONS. 7 



port that a prism of chromate of lead, owing to the nature of the sub- 

 stance, will not enable him to determine the indices, as the whole spec- 

 trum is confused, no lines visible, and the violet end totally absorbed. 



In the identification of certain of the standard raj's of Fraunhofer 

 some discrepancy appeared to exist between different representations. 

 The author's attention is now directed to this point, among others 

 connected with a more accurate repetition of his former approximate 

 determinations of refractive indices, on which he is now engaged. 



He wishes also to draw attention to questions connected with the 

 application of photometry to the theory ; especially to that referring to 

 the power of the eye to judge of the equalization of lights, and the in- 

 fluence which the illumination of one space has upon that of another in 

 juxtaposition. To show how great the uncertainty Ls, the following 

 very simple experiment may be referred to. On receiving the rays of 

 a candle on a white screen, and intercepting a portion of them by a 

 clear plate of glass, the eye can recognise no difference in the illumi- 

 nation of the covered part. Yet, from both the first and second sur- 

 faces of the glass, there is a copious reflection. 



With regard to the mathematical theory, he alludes to the important 

 researches of Mr. Tovey, especially those on elliptic polarization. All 

 the preceding investigations for integrating the differential equations for 

 waves, including the dispersion, have proceeded on the supposition that 

 certain terms vanish. This appears essential to the general solution. 

 Mr. Tovey has, however, shown, that if those terms do not vanish, we 

 have still a particular solution : and this applies to the case of light 

 elliptically polarized. This case is absolutely excluded in the former 

 investigations, which are therefore imperfect. The author has endea- 

 voured to clear up some points connected with this inquiry. Upon the 

 evanescence or non-evanescence of these terms simply depends the el- 

 liptic, circular, or rectilinear character of the vibrations. Corresponding 

 to these mathematical conditions, are those of the arrangement of the 

 aetherial molecules in the medium, or part of the medium, where the 

 polarization is communicated. He has pointed out the connexion be- 

 tween these views and the investigations of Prof. MaccuUagh, in which 

 that gentleman connects with certain equations of motion the elliptic 

 polarization in quartz, by which Mr. Airy had explained the results 

 and laws of M. Biot. 



On an Ocular Parallax in Vision, and on the Law of Visible Direc- 

 tion. By Sir D. Brewster, K.H., F.R.S. 



The honour of suggesting or illustrating the law of visible direction 

 belongs, said Sir David Brewster, to Dechales, Porterfield, and Reid. 

 D'Alembert, in his " Doutes sur dift'erentes questions d'Optique*," 

 maintains that the action of light upon the retina is conformable to the 



* Opuscules Mathcmatiques, torn. i. p. 266, 268. 



