Sir D. Brewster's Observations on M. Kudbev^s Memoir. 7 



which stands at 0'035 of the scale ; and upon comparing it 

 with the place assigned to Iceland crystal by Dr. Wollaston, 

 I was surprised to find that he placed its dispersive power 

 very considerably above water, and even above diamond. 

 This unexpected difference between the two measures in- 

 duced me to repeat the experiments, not only with other 

 prisms of the Iceland spar, but also with other standard 

 prisms of flint and crown glass. These new results served 

 only to confirm the accuracy of the first experiment, and to 

 strengthen my suspicion that Dr. Wollaston had committed 

 a mistake. As this reasoning, however, was founded on the 

 assumption which both Dr. Wollaston and I had made, — that 

 the spar had only one dispersive power, — I resolved to mea- 

 sure the dispersive power of the extraordinary refraction. 

 This new value having turned out to be greater than that of 

 water, I immediately saw that Dr. Wollaston had measured 

 the colour of the greatest refraction, while I had measured 

 the colour of the least; and that this remarkable mineral, 

 which had so long perplexed philosophers by its double re- 

 fraction, possessed the no less extraordinary and inexplicable 

 property of two dispersive powers. In subjecting to exami- 

 nation other crystals that afforded double images, — such as 

 carbonate of strontites, carbonate of lead, and chromate of 

 lead, — I found that every separate refraction possessed a se- 

 parate dispersive power. This general law, though not re- 

 pugnant to any optical phaenomena, is still of such a nature, 

 that it could not have been inferred a priori from any rela- 

 tion v/hich is known to subsist between the refractive and 

 dispersive powers. No person, indeed, has even conjectured 

 that a double dispersive should accompany a double refractive 

 power : and if we were to reason in this case from an analogy 

 founded on experiment, — an analogy, too, which is by no 

 means remote, we should certainly conclude, contrary to the 

 fact, that the greatest refractive power would be accortipanied 

 with the least power of dispersion. In all the minerals in which 

 a metal is the principal ingredient, those which have the 

 greatest refractive density have also the greatest faculty of 

 producing colour; while in all the precious stones a high re- 

 fractive power is attended with a low power of dispersion. 

 This remarkable property of a double dispersion, therefore, is 

 contrary to the general results indicated by experiments; and 

 though it appears to exclude some of the theories by which 

 a double refraction has been explained, it certainly adds 

 another to those numerous difficulties with which philosophy 

 has yet to struggle, before she can reduce to a satisfactory 



