65 



The (liiference between absorption and dispersion is, that, in the 

 former case, the absorbed light is extinguished ; while, in the latter, 

 the dispersed light is visible. Hence, if, in powerfully absorbing 

 bodies, the absorbed light could be rendered visible, we should have 

 the phenomena of epipolised light. 



3. On the polarisation of dispersed light. Sir J. Herschel had 

 failed to detect the polarisation of epipolised light ; but the author, 

 by using a condensed sunbeam, discovered that the dispersed beam 

 was polarised, partly in the plane of reflexion, and partly in a 

 peculiar manner ; which the author calls quaquaversus polarisation, 

 like that effected by a congeries of small doubly-refracting crystals, 

 having their axes in all directions. Ho afterwards discovered in- 

 stances in which each of these kinds of polarisation was found alone. 

 These experiments have led to more extended investigations, to be 

 subsequently conmiunicated. The remainder of the section contains 

 a minute description of the beautiful phenomena of the polarisation 

 of dispersed light. The author thinks it probable that the study of 

 these phenomena may throw much light on the internal structure of 

 the substances exhibiting them. 



4. On the causes of the decomposition and internal dispersion of 

 light. The author ascribes the pltenomena (which do not, in fluor- 

 spar, belong to the species, but only to certain varieties) to irregular 

 crystallisation, and the successive deposition of strata of different 

 refractive and dispersive power. He doM not, at present, offer any 

 theory of the cause in liquids. 



2. A few Remarks suggested by Professor Forbes's Descrip- 

 tion of the effects of Glacial Action among the Cuchullin 

 Hills, and Mr Maclaren's views of the facts observed by 

 him at the Gareloch. By Sir G. S. Mackenzie, Bart. 



The object of this paper was to shew, that if it be assumed that 

 glaciers had actually existed among the Cuchullin Hills, or in other 

 parts of this country, one of two conditions must necessarily have 

 been present — either a climate much colder than the existing 

 climate of Scotland, or a higher position of the land where the 

 supposed marks of glaciers are seen. With respect to the first con- 

 dition, the permanency of the relative positions of the heavenly 

 bodies, and of the inclination of the earth's axis, and the adaptation 

 to existing clim<ates of animal and vegetable lite, were appealed t«> 



VOL. n. n 



