195 



Certain curious differences appear to follow the externality or in- 

 ternality of the shell. 



An external shell in a mollusk with a hsemal flexure, e. g. Atlanta, 

 has its columellar axis helow the aperture. 



An external shell in a mollusk with a neural flexure, e. g. Nautilus, 

 has its columellar axis above the aperture. 



An internal shell in a mollusk with a neural flexure, has its colu- 

 mellar axis helow the aperture, e.g. Spirula, Clausilia, Helix. 



In the course of the memoir the author incidentally introduces a 

 number of new, and, as he believes, important facts, with regard to 

 the nervous, circulatory and urinary systems ; and describes at 

 length the mechanism of the " tongue" and a^J organ similar to the 

 "crystalline style" of bivalves, found in the Strombidee. 



The following papers were also read :—- 



1. *' On the Change of Refrangibility of Light." By George G. 

 Stokes, Esq., M.A., F.R.S., Lucasian Professor of Mathematics, 

 Cambridge. Received May 11, 1852. 



The author was led into the researches detailed in this paper by 

 considering a very singular phenomenon which Sir John Herschel 

 had discovered in the case of a weak solution of sulphate of quinine, 

 and various other salts of the same alkaloid. This fluid appears 

 colourless and transparent, like water, when viewed by transmitted 

 light, but exhibits in certain aspects a peculiar blue colour. Sir 

 John Herschel found that when the fluid was illuminated by a beam 

 of ordinary daylight, the blue light was produced only throughout a 

 very thin stratum of fluid adjacent to the surface by which the light 

 entered. It was unpolarized. It passed freely through many inches 

 of the fluid. The incident beam, after having passed through the 

 stratum from which the blue light came, was not sensibly enfeebled 

 nor coloured, but yet it had lost the power of producing the usual 

 blue colour when admitted into a solution of sulphate of quinine. A 

 beam of light modified in this mysterious manner was called by 

 Sir John Herschel epipolized. 



Several years before Sir David Brewster had discovered in the 

 case of an alcoholic solution of the green colouring matter of leaves 

 a very remarkable phenomenon, which he has designated as internal 

 dispersion. On admitting into this fluid a beam of sunlight con- 

 densed by a lens, he was surprised by finding the path of the rays 

 within the fluid marked by a bright light of a blood-red colour, 

 strangely contrasting with the beautiful green of the fluid itself when 

 seen in moderate thickness. Sir David afterwards observed the 

 same phenomenon in various vegetable solutions and essential oils, 

 and in some solids. He conceived it to be due to coloured particles 

 held in suspension. But there was one circumstance attending the 

 phenomenon which seemed very difiicult of explanation on such 

 a supposition, namely, that the whole or a great part of the dispersed 

 beam was unpolarized, whereas a beam reflected from suspended 



