Page:Proceedings of the Royal Society of London Vol 1.djvu/474

Rh Along with the polarized light, Dr. Brewster also observed a faint nebulous light not polarized, which he also ﬁnds in transmission through carnelian 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 troneg than the diamond. Phosphorus, he adds, stands higher than has been supposed, being 2‘224, and then native sulphur next in order 2115.

The dispersive power of chromate of lead is observed to exceed that of other substances in a still greater proportion than its refractive power, being more than ten times as great as that of the denset ﬂint-glass, and ﬁfteen times as high as that of water.

The concluding section of the author‘s letter relates to the existence of two diapersive powers in all doubly refracting media. Mr. Cavallo, and others, have already observed, that the dispersion occasioned 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, i 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 diﬂiculties that are to be surmounted in reducing to any general rules those capricious phenomena. exhibited by light in its passage through transparent bodies.

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 I slightly concave. In a few years more the same glass was not sufﬁciently powerful; but yet his degree of short-sightedness was so inconsiderable, that he yielded to the dissunsion of his friends from