388          ON  THE   CHANGE  OF  RRFRANGIBILITY  OF  LIGHT.
sion, exhibited when heated a blue phosphorescent light; but this appears to have been merely a casual coincidence*.
On the Cause of True Internal Dispersion, and of Absorption.
226.    In considering the cause of internal dispersion, we may I think at once discard all supposition of reflexions and refractions of the vibrations of the luminiferous ether among the ultimate molecules of bodies.    It seems to be quite contrary to dynamical principles to suppose that any such causes should be adequate to account for the production of vibrations of one period from vibrations of another.
All believers, I suppose, in the undulatory theory of light are agreed in regarding the production of light in thu first instance as due to vibratory movements among the ultimate molecules of the self-luminous body. Now in the phenomenon of internal dispersion, the sensitive body, so long as it is under the influence of the active light, behaves as if it were self-luminous. Nothing then seems more natural than to suppose that the incident vibrations of the luminiferous ether produce vibratory movements among the ultimate molecules of sensitive substances, and that, the molecules in turn, swinging on their own account, produce vibrations in the luminiferous ether, and thus cause the sensation of light. The periodic times of these vibrations depend upon the periods in which the molecules are disposed to swing, not upon the periodic time of the incident vibrations.
227.    But in the very outset of this theory an objection will probably be urged, that it is quite as much contrary to dynamical principles to suppose the periodic time of tin* ethereal vibrations capable of being changed through the intervention of ponderable molecules as without any such machinery")'.    The answer to this objection is, that such a notion depends altogether on the applicability of a certain dynamical principle relating to indefinitely small motions, arid that we have no right to regard the molecular vibra-
* Report of the Meeting of the liritiah Association at AVimtsM- in 18M, p. 11.
f [The attempt here made to account for the lowering of rcfranKibilit.v him lontf since been given up, and I have been led to adopt what KeeniH to im» a far more probable explanation. See Addition to this paper, p. 410.]