The older view of olfaction—and many modern scientists, as we see, still adhere to it—is that the odorous molecule acts as a chemical reagent upon the olfactory hairs. And there is something to be said for this opinion.
To begin with, no one doubts nowadays that odours are material. They pass through the air as vapours, and they are known to travel miles on the wind. That is to say, apart from those hypothetical varieties of odour (if we can call them odour at all) discussed by Fabre earlier in this book, odours do not emanate from a point and disperse in all directions as light and sound do. Why then drag in the ether? Is it not more probable that the odorous molecule acts on the olfactory hairs by direct material contact, and that it sets up chemical changes in them?
We are asked to believe that the ultra-violet rays of odour stimulate the olfactory hairs as visible light-rays stimulate the retina. But it must not be forgotten that in the eye those rays may induce first of all chemical changes in the retina, just as they would act on the silver salt of a photographic plate, and that it may be by these changes that the retina is stimulated.
In the phenomenon of olfactory exhaustion, as we said in our first chapter, we have a circumstance which suggests the presence of some chemical reagent in the olfactory area.
It may be, of course, that in the nose as well as in the eye the process is a combination of chemical and physical changes. And in any case we are here dealing with that obscure region where chemistry and physics meet and mingle.
We have now come to the end of our discourse upon the theories of odour, and it must be confessed that we are still very much in the dark as to the nature of the odorous, and as to the manner in which it excites the olfactory organ to activity.
Still more mysterious, however, is the process by which the physical quality of odour becomes the sensation of the mind we call smell.
The transmutation of a physical quality into a sensation is indeed the great mystery of all our senses. Olfaction is not the only one before which we throw up our hands, and this in spite of the detailed and voluminous information which modern physiology, neurology, and psychology place at our disposal, perhaps less in spite of this information than because of it, seeing that the further our knowledge extends the wider seems the unknown realm beyond. Our science is an ever-expanding sphere, no doubt, but it is expanding into the infinite.
How is it that the rhythmic vibration of matter becomes what we call “sound,” or the rhythmic vibration of the ether “light”?
How does the physical pass into and become part of the psychic?