t 270 ON THE CHANGE OF REFRANGIBILITY OF LIGHT.
j
• J theless the mystery was by no means cleared up; rather, we were
,' prepared to expect something of the same sort in other instances
\l of internal dispersion. In fact, the mystery consisted, not m
','! the narrowness of the stratum from which most of the blue light
j' came, but in the circumstance that it was possible for light, by
;!f 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.
l 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 5 /; 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 are 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 dispensed 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,
* 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