1853.] OF THE ROYAL INSTITUTION. 261 
tion band in the red, withdrawn, as they certainly were, from the 
incident beam, were given out in all directions, instead of being 
absorbed in the manner usual in coloured media. It might be sup.- 
posed that the incident vibrations of the luminifersus ether 
generated synchronous vibrations in the ultimate molecules, and 
were thereby exhausted, and that the molecules in turn became 
centres of disturbance to the ether. The general analogy between 
the phenomena exhibited by the solutions of chlorophyll and of 
quinine would lead to the expectation of absorption bands in the 
light transmitted by the latter. If these bands were but narrow, 
the light belonging to them might not be missed in the transmitted 
beam, unless it were specially looked for; and the beam might be 
thus ‘‘ epipolized,” without, to ordinary inspection, being changed 
in its properties in any other respect. But on subjecting the light 
to prismatic analysis, first with the naked eye, and then with 
a magnifying power, no absorption bands were perceived. 
A little further reflection shewed that even the supposition of the 
existence of these bands would not alone account for the phenomenon. 
For the rays producing the dispersed light, (if we confine our 
attention to the thin stratum in which the main part of the dis- 
persion takes place,) are exhausted by the time the incident light 
has traversed a stratum the fiftieth of an inch thick, or there- 
abouts, whereas the dispersed rays traverse the fluid with perfect 
freedom. This indicates a difference of nature between the blue- 
producing rays and the blue rays produced. Now, as the Lecturer 
stated, he felt very great confidence in the principle that the nature 
of light-is completely defined by specifying its refrangibility and its 
state as to polarization. The difference of nature, then, indicated 
by the phenomenon, must be referred to a difference in one or other 
of these two respects. At first he took for granted that there could 
be no change of refrangibility. The refrangibility of light had 
hitherto been regarded as an attribute absolutely invariable.* To 
suppose that it had changed would, on the undulatory theory, be 
equivalent to supposing that periodic vibrations of one period could 
give rise to periodic vibrations of a different period, a supposition 
presenting no small mechanical difficulty. But the hypotheses 
which he was obliged to form on adopting the other alternative, 
namely, that the difference of nature had to do with the state of 
polarization, were so artificial as to constitute a theory which ap- 
peared utterly extravagant. He was thus led to contemplate the 
possibility of a change of refrangibility. No sooner had he dwelt 
* It is true that the phenomenon of phosphorescence is in a certain sense 
an exception; but the effect is in this case a work of time, which seems at once 
to remove it from all the ordinary phenomena of light, which, as far as sense can 
judge, take place instantaneously. It is true that there now appears a close analogy 
in many respects between true internal dispersion and phosphorescence. But 
while the nature of epipolized light remained yet unexplained, there was nothing 
in the former phenomenon to point to the latter, 
