PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 465 
in other instances of internal dispersion. In fact, the mystery consisted, not in the 
narrowness of the stratum from which most of the blue light came, but in the cir- 
cumstance that it was possible for light, by passing across such a stratum, to be 
deprived of the power of producing the same effect again, without, apparently, being 
altered in any other respect. 
4. To one who regards light as a subtle and mysterious agent, of which the laws 
indeed are in a good measure known to us, but respecting the nature of which we are 
utterly ignorant, the phenomenon might seem merely to make another striking 
addition to the modes of decomposition with which we were already acquainted. 
But in the mind of one who regards the theory of undulations as being for light what 
the theory of universal gravitation is for the motions of the heavenly bodies, it was 
calculated to excite a much more lively interest. Whatever difficulty there might be 
in explaining how the effect was produced, we ought at least to be able to say what 
the effect was that had been produced; wherein, for example, epipolized light differed 
from light which had not undergone that modification. 
In speculating on the nature of the phenomenon, there is one point which deserves 
especial attention. Although the passage through a thickness of fluid amounting to 
a small fraction of an inch is sufficient to purge the incident light from those rays 
which ai’e capable of producing epipolic dispersion, the dispersed rays themselves 
traverse many inches of the fluid with perfect freedom. It appears therefore that the 
rays producing dispersion are in some way or other of a different nature from the 
dispersed rays produced. Now, according to the undulatory theory, the nature of 
light is defined by two things, its period of vibration, and its state of polarization. 
To the former corresponds its refrangibility, and, so far as the eye is a judge of colour, 
its colour *. To a change, then, either in the refrangibility or in the state of polari- 
zation we are to look for an explanation of the phenomenon. 
5. Regarding it at first as an axiom that the dispersed light of any given refrangi- 
bility could only have arisen from light of the same refi-angibility contained in the 
incident beam, I was led to look in the direction of polarization for the required 
change in the nature of the light. Since a fluid has no axes, circular polarization is 
* It has been maintained by some philosophers of the first eminence that light of definite refrangibility may 
still be compound, and though no longer decomposable by prismatic refraction might still be so by other means. 
I am not now speaking of compositions and resolutions depending upon polarization. It has even been 
suggested by the advocates of the undulatory theory, that possibly a difference of properties in lights of the 
same refrangibility might correspond to a difference in the law of vibration, and that lights of given refrangibility 
may differ in tint, just as musical notes of given pitch differ in quality. Were it not for the strong conviction 
I felt that light of definite refrangibility is in the strict sense of the word homogeneous, I should probably have 
been led to look in this direction for an explanation of the remarkable phenomena presented by a solution of 
sulphate of quinine. It would lead me too far from the subject of the present paper to explain the grounds of 
this conviction. I will only observe that I have not overlooked the remarkable effect of absorbing media in 
causing apparent changes of colour in a pure spectrum ; but this I believe to be a subjective phenomenon, de- 
pending upon contrast. 
3 o 2 
