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. Finally, we have the question of the olfactory pigment to consider, and in this matter we cannot do better than follow the exposition of William Ogle, an English physician who wrote as long ago as 1870. As will be seen, he forestalls the modern undulatory theory of olfaction in a remarkable manner. Ogle contends that the presence of pigment must be of great importance in the function for the following reasons: First, the epithelium of the olfactory region is pigmented, while that of the rest of the nasal chamber and sinuses is devoid of colouring matter. Secondly, there seems to be some correspondence between the degree of pigmentation and the acuteness of smell, as the following facts suggest:— In macrosmatic animals, such as the dog, cat, fox, sheep, and rabbit, pigmentation extends over a larger space and is darker in tint than in man. In these animals also the mucus covering the olfactory area of the nose is itself pigmented.