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.
In the same way, Heyninx and others argue that the odorous vapour is composed of molecules which are vibrating with a period equal to that of the light-rays they absorb.
Moreover, since the position of the absorption-band in the photograph varies, lying in some cases nearer to the visible violet and in others further away from it, and since this position varies with the particular fundamental odour employed, it is suggested that not only do the molecules vibrate with a period equal to that of the ultra-violet rays they absorb, but as this vibration varies in rate, so it is to this variation that we must ascribe the differences in odours. This is analogous, of course, to the appreciation of colour by the eye. One odorous molecule, that is to say, like the colour red, having a slower rate of vibration, will give rise to one kind of smell; another, like the colour yellow, with a more rapid rate, will give rise to another kind of smell, and so on for all the fundamental odours. Heyninx, indeed, goes so far as to fix the position in the olfactory gamut of all fundamental odours, and to base upon it the classification we have already considered.
It is supposed, that is to say, that the vibrations of the odorous molecule set up undulations in the ether, and that it is those ethereal undulations that stimulate the olfactory hairs, just as ethereal undulations emanating from a luminous source stimulate the retina.
There is one great difference, however, between light and odour, a difference admitted, we may mention, by the supporters of the undulatory theory, but not emphasised by them. The difference is this: in the case of visible light the ethereal undulations emanate from a source at a distance (it may be like starlight at an enormous distance) from the sensory end-organ, whereas in the case of odour the undulation is supposed to be generated by the odorous molecule in close proximity to the end-organ.
The theory makes no attempt to explain how the olfactory hairs respond to these hypothetical ethereal waves.