The physical theory of odour, then, refers that quality to the vibration of the molecule. It suggests that the molecules of an odorous body passing in the gaseous or, in fishes, the liquid state into the olfactory region of the nose, are there received by the film of mucus in which the olfactory hairs lie, and stimulate these hairs by their molecular vibration. No chemical change is supposed to take place, only, as it were, a mechanical stimulation, comparable to the mechanical stimulation of the retina by the waves of light. A recent development of the theory which we owe to Heyninx, a Belgian scientist, brings the process very closely into harmony with what occurs in the eye. According to this authority, olfaction is in reality a perception of ethereal undulations of the same character as the undulations of light, these undulations being provoked by the intra-molecular vibrations of the odorous vapour in the nasal mucus and transmitted to the olfactory hairs not by immediate contact, but through the medium of the ether. We owe this last suggestion to the curious fact, but recently discovered, that many odorous substances (in their gaseous form in the air) absorb the rays of ultra-violet light. In order to make clear what this means, we must say a preliminary word regarding the spectrum and spectrum analysis. The passage of a beam of white light through a glass prism breaks it up into its component parts, beginning with red, then orange, yellow, green, blue, and ending with violet. Beyond the violet end of the spectrum we know there are rays invisible to us, but capable of acting on a photographic plate. These are called the ultra-violet rays. In like manner, beyond the red end of the spectrum we know there are also rays, likewise invisible to us, but perceptible by our tactile sense as heat. These are called the infra-red rays. Now, the rate of vibration of all these different rays, visible and invisible, has been estimated, and they increase in frequency from the infra-red, which are the slowest, to the ultra-violet, which are the most rapid. As we have already said, it has recently been shown that the odorous vapours absorb certain ultra-violet rays. That is to say, when the beam of light is directed through a chamber containing the odorous vapour before entering the prism, what are known as absorption-bands—vertical black lines in the white—appear in the photograph of the spectrum. Similar lines are seen, as a matter of fact, in the visible spectrum of sunlight, and as these correspond in position with the spectrum given by chemical elements in an incandescent gaseous state, it is supposed that they are produced by the absorption of the corresponding light-rays by these gases in the solar atmosphere. The physical explanation given of this phenomenon is that the molecules of the gas in the sun absorb such light-rays as are equal in rate of vibration to the rate of their own vibrating molecule.