ON THE CHANGE OF KEFRANGIBILITY OF LIGHT. 351
if examined are too few to justify me in entering into detail. ||
Besides, the absorption bands due to protoxide of uranium do not belong properly to my subject, the compounds of this oxide, so far as I have examined, being insensible.
Appearance of highly Sensitive Media in a Beam from which the ||
Visible Rays are nearly excluded.
164. When a large beam of sunlight is reflected horizontally into a darkened room, and transmitted through an absorbing medium, placed in the window, of such a nature as to let pass only the feebly illuminating rays of high refrangibility and the invisible rays beyond, various sensitive media have a very strange and unnatural appearance when placed in the beam, on account of the peculiar softness of the dispersed light with which the media appear as it were self-luminous, and the almost entire absence of strong light reflected from convexities. Among substances eminently proper for this experiment, may be mentioned a solution of the bark of the horse-chestnut*', or of sulphate of quinine, or of stramonium seeds, a decoction of madder in a solution of alum, and above all, ornamental articles of canary glass. The appearance of a specimen of yellow uranite was curiously altered by this mode of examination. By daylight the mineral appeared much of the same colour as the stone in which it was imbedded, but when placed in a beam such as that above mentioned the uranite was strongly luminous, while the stone remained dark.
Natural Crystals.
165. Of natural crystals I have hitherto examined only a small number. For a long time I was occupied almost exclusively with vegetable products, the mineral kingdom not appearing promising. However, I have found internal dispersion in certain specimens of apatite, aragonite, chrysoberyl, cyanite, and topaz. In all these cases the dispersion appeared due, as in the case of
[* A solution which answers admirably, and is very easily prepared, is obtained by adding to a decoction when cold a suitable quantity of alum or a ferric salt, precipitating by ammonia, and filtering. The powerful fluorescence of the solution is due to mixed aesculin and fraxin.]