272 ON THE CHANGE OF BEFRANGIBILIIT OF LIGHT.
Nevertheless it seemed to me less improbable that the refrangi-bility should have changed, than that the undulatory theory should have been found at fault. And when I reflected on the extreme simplicity of the whole explanation if only this one supposition be admitted, I could not help feeling a strong expectation that it would turn out to be true. In fact, we have only to suppose that the invisible rays beyond the extreme violet give rise by internal dispersion to others which fall within the limits of refrangibility between which the retina of the human eye is affected, and the explanation is obvious. The narrowness of the blue band observed by Sir John Herschel would merely indicate that the fluid, though highly transparent with regard to the visible rays, was nearly opaque with regard to the invisible. According to the law of continuity, the passage from almost perfect transparency to a high degree of opacity would not take place abruptly; and thus rays of intermediate refrangibilities might produce the blue gleam noticed by Sir John Herschel, or the blue cylinder, or rather cone, observed by Sir David Brewster. We should thus, too, have an immediate explanation of a remarkable circumstance connected with the blue band, namely that it can hardly be seen by strong candle-light, though readily seen by even weak daylight. For candle-light, as is well known, is deficient in the chemical rays situated beyond the extreme violet. 7. My first experiments were made with coloured glasses. *j(! A test tube was about half filled with a solution consisting of
,!' disulphate of quinine dissolved in 200 times its weight of water
|, acidulated with sulphuric acid. The tube, having been first
|ij covered with black paper, with the exception of a hole by which
"Il the light might enter, was placed in a vertical position in front of
,!• a window, the hole being turned towards the light. On looking
I down from above, in a direction nearly parallel to the surface
'- of the glass, a blue arc was well seen, extending only a very
j'i short distance into the fluid, and situated immediately behind
the hole. As this arc, though extremely distinct, was not of course what could be called brilliant, I did not at first venture, for the experiment I had in view, to use any but pale glasses. I Having no direct means of determining which were opaque with
regard to the invisible rays situated beyond the extreme violet, I sought among a collection of orange, yellow, and brown glasses, which, from transmitting mainly the less refrangible rays, seemed