Dr. Brewster on some Properties of Light, 1 09 



Iceland crystal ; but it is obvious, from a simple inspection of 

 the table, that these are measures of the dispersion or quantity 

 of colour, and not of the dispersive power of the substances. 

 The measures in the table alluded to, vvith the exception of 

 one or two, are so completely incompatible with those taken 

 by Dr. Wollaston and myself, that I can scarcely believe 

 that the experiments were ever made. 



The singular property of a double dispersive power, while 

 it seems to exclude some of the theories by which the double 

 refraction has been explained, adds another to those numerous 

 difficulties with which philosophy has yet to struggle, before 

 she can reduce to a satisfactory generalization those anomalous 

 and capricious phenomena which light exhibits in its passage 

 through transparent bodies. 



I have the honour to be, 

 dear Sir, 

 your most obedient humble servant, 



DAVID BREWSTER, 



Edinburgh, 13, Duke-strect, 

 December 19, 1812. 



To Sir H. Davy, LL.D. F, R. S, 



