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 ratc 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 80 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
