Dr, Brewster on some Properties of Light. 107 



is next to diamond in refractive power, and that the three 

 simple inflammable substances have their refractive powers 

 in the order of their inflammability. Dr. Wollaston has 

 placed phosphorus below horn and flint glass,* but I am con- 

 fident that this distinguished philosopher, to whom the physi- 

 cal sciences are so deeply indebted, will find, upon making the 

 experiment with prisms or lenses, that I have assigned the 

 right place to that remarkable substance. The difference be- 

 tween the extreme dispersive powers in the second table is 

 very remarkable, and the result for oil of cassia indicates in 

 that body the existence of some ingredient, which chemical 

 analysis has not been able to detect. 



4. On the Existence of two dispersive Powers in all doubly 



refracting Crystals. 



It has been long known, and it is indeed obvious, from a 

 simple inspection of the images formed by a prism of Iceland 

 crystal, that the one image is more coloured than the other, 

 or that the actual dispersion of the one refraction is greater 

 than the dispersion of the other, in the same manner as the 

 dispersion of a prism of flint glass with a refracting angle of 

 12 degrees, is greater than the dispersion of a prism of the 

 same glass with an angle of only 10 degrees. 



Dr. Wollaston, who was the first person that examined 

 the subject of dispersive powers with philosophical accuracy, 

 makes the dispersive power of Iceland spar considerably above 

 water, and even above diamond. Upon repeating this expe- 

 riment, with the least refracted image, I found the dispersive 



• Dr. WoLLASTOK is satisfied that his original estiiiiate was erroneous^ and that 

 Dr. Brxwster's determination is very near the truth. H. D. 



P 2 



