It appears from the analytical investigation of this phenomenon that the vibrations of the fluorescent substance are analogous to those of a sonorous body, as a bell or musical cord, which give the fundamental note and its harmonics. Now since there is a reciprocal action between the molecules of matter and light, when the light of the sun is absorbed by a substance capable of fluorescence, it puts the whole of its molecules into vibrations the same as its own, analogous to the fundamental note, while at the same time a certain number of molecules take more rapid vibrations exactly like the harmonics. The latter form new centres of light throughout the substance, which impart their vibrations to the ethereal medium around, and constitute fluorescence or degraded light. For example, in the experiment that has been described, the blue light imparted its own vibrations to _all_ the molecules of the canary-glass, and also more rapid vibrations to a certain number of them. All of the blue rays were excluded by the yellow glass held before the eye; but it was pervious to the rays emanating in more rapid vibrations from the smaller number of molecules, which thus became really new centres of light, different from the sun’s light, though owing to it; the one celestial, the other terrestrial; and the latter vibrations being more rapid than those of the blue light, their refrangibility was less, and therefore their colour lower in the prismatic scale. Mr. Power computed from his formulæ, that fluorescent light is produced by undulations which are a major or minor third below the pitch of the general vibration of the medium—that is to say, below the vibrations which the whole molecules of the body most readily assume.