454 



Along with the polarized light, Dr. Brewster also observed a faint 

 nebulous light not polarized, which he also finds in transmission 

 through cornelian and chalcedony, and thinks it important as leading 

 to a satisfactory theory of polarization. 



The next observation of the author relates to the high refractive 

 power of chromate of lead, which, he remarks, is greater than that 

 of any other body hitherto recorded ; and upon its double refraction, 

 which, he says, is so enormous, that the deviation of the extraordinary 

 ray is more than thrice that produced by Iceland-spar. 



The index of refraction assigned by Dr. Brewster to chromate of 

 lead, is 2'926, and along with it he names realgar, of which the in- 

 dex is 2-510, as another substance that refracts more strongly than 

 the diamond. Phosphorus, he adds, stands higher than has been sup- 

 posed, being 2'224, and then native sulphur next in order 2' 115. 



The dispersive power of chromate of lead is observed to exceed 

 that of other substances in a still greater proportion than its refrac- 

 tive power, being more than ten times as great as that of the densest 

 flint-glass, and fifteen times as high as that of water. 



The concluding section of the author's letter relates to the exist- 

 ence of two dispersive powers in all doubly refracting media. Mr. Ca- 

 vallo, and others, have already observed, that the dispersions occa- 

 sioned by the two refractions of Iceland-spar are not equal. Dr. 

 Brewster observes that this is general to all, and he undertakes to 

 assign the proportion of these two powers in different substances. 



In chromate of lead the dispersive power manifested in the rays" 

 ordinarily refracted, is double that of rays obliquely refracted ; and 

 in Iceland-spar the disparity is nearly as great. 



The existence of a double dispersive power, it is observed by the 

 author, instead of assisting in the explanation of other properties, 

 only adds one to the numerous difficulties that are to be surmounted 

 in reducing to any general rules those capricious phenomena exhi- 

 bited by light in its passage through transparent bodies. 



An Appendix to Mr. Ware's Paper on Vision. By Sir Charles Blag- 

 den, F.R.S. Read February 4, 1813. [Phil. Trans. 1813,;?. 110.] 



The author remarks, that Mr. Ware's observations with regard to 

 short-sightedness, being in general merely the consequence of habit 

 acquired at an early age, is conformable with his own experience in 

 general, and that he himself is a particular instance of natural long- 

 sightedness gradually converted into confirmed short sight. He very 

 well remembers first learning to read, at the common age of four or 

 five years, and that at that time he could see the usual inscriptions 

 across a wide church ; but that at the age of nine or ten years he 

 could no longer distinguish the same letters at the same distance, 

 without the assistance of a watch-glass, which has the effect of one 

 slightly concave. In a few years more the same glass was not suf- 

 ficiently powerful ; but yet his degree of short-sightedness was so in- 

 considerable, that he yielded to the dissuasion of his friends from 



