466 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
the only kind which can here come into play. As some fluids are doubly refracting, 
transmitting right-handed and left-handed circularly polarized light with different 
velocities, so, it might be, this fluid was doubly absoi-bing, absorbing say right-handed 
circularly polarized light of certain refrangibilities with great energy, and freely 
transmitting left-handed. The right-handed light, absorbed, in the sense of with- 
drawn from the incident beam, might have been more strictly speaking scattered, 
and thereby depolarized. The common light so produced would be equivalent to two 
streams, of equal intensity, one of right-handed, and the other of left-handed circu- 
larly polarized light. Of these the latter would be freely transmitted, while the 
former would be scattered anew, and so on. Yet this hypothesis, sufiiciently impro- 
bable already, was not enough. New suppositions were still required, to account for 
the circumstance that an epipolized beam, when subjected to prismatic analysis with 
a low magnifying power, exhibited no bands of absorption in the region to which, as 
regards their refrangibility, the dispersed rays principally belong ; so that altogether 
this theory bore not the slightest semblance of truth. 
6. I found myself thus fairly driven to suppose that the change of nature consisted 
in a change of refrangibility. From the time of Newton it had been believed that 
light retains its refrangibility through all the modifications which it may undergo. 
Nevertheless it seemed to me less improbable that the refrangibility should have 
changed, than that the undulatory theory should have been found at fault. And 
when I reflected onthe 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 expla- 
nation 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 opake 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 imme- 
diate 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 situ- 
ated beyond the extreme violet. 
7. My first experiments were made with coloured glasses. A test tube w-as 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 
covered with black paper, with the exception of a hole by which the light might 
enter, was placed in a vertical position in front of a window, the hole being turned 
