io8 Dr, Brewster on some Properties of Light. 



power, or the value of -^^ , to be 0.026 very considerably below 



water, which stands at 0.035 of the scale, and I therefore 

 concluded that Dr. Wollaston had examined the greatest 

 refraction, while I had examined the least, and that the vast 

 discrepancy between our measures arose from the existence 

 of a double dispersive power. This conclusion was confirmed 

 by determining the dispersive power of the greatest refraction, 

 which coincided exactly with the order assigned to it by Dr. 

 Wollaston. 



The dispersive powers, which I have obtained for other 

 doubly refracting crystals, such as carbonate of strontites, 

 carbonate of lead, and chromate of lead, have confirmed this 

 result, and establish the general law, that each refraction of 

 crystals which give double images is accompanied with a 

 separate dispersive power. The double dispersive powers of 

 these bodies are given in the following table. 



Chromate of lead (gr. refr. ) estimated at 0.400 



Ditto Ditto must exceed 0.296 



Ditto (least refr.) - - 0.26a 



Carbonate of lead ( gr. refr. ) - +0.091 



Ditto ( least refr. ) - - 0.066 



Carbonate of strontites ( gr. refr. ) - 0.046 



Ditto ( least refr. ) - 0.027 



Calcareous spar (gr. refr.) - - 0.040 



Ditto (least refr.) - 0.026 



In a table of refractive powers, published by the late Mr. 

 Cavallo, he has given, from other authors, the dispersions, or 

 the dissipations as he calls them, of a few substances, and he 

 has annexed a different dispersion to the two refractions of 



