say that the undulatory theory of olfaction is, that an odorivector gives off in the form of vapour (in the aerial medium) extremely attenuated portions of its substance, too minute to be weighed, and that this vapour, disseminated through the air, enters the nose in respiration, and, being wafted up into the olfactory region, is received by the mucus bathing the olfactory hairs, where, in virtue of the ultra-violet radiations which proceed from its molecules and are modified by the olfactory pigment, it acts on the hairs, setting up changes (it may be also undulatory in nature) in them and in their cells, which changes are transmitted thence by the olfactory nerves to the neurones or nerve-cells of the olfactory bulb (or lobe) of the brain.
The undulatory theory of olfaction, then, as will be evident to the reader, has a good deal in its favour. And in addition to what we have already said of it as accounting for the absorption by odorous vapours of ultra-violet rays, and as giving a hint regarding the function of pigment in the olfactory area, there are also a number of other phenomena which it seems to explain. We have seen, for example, how one odorivector, such as musk or civet, may have the property of enhancing the power of another, and this is a property which is characteristic also of certain luminous conditions (fluorescence, lumino-luminescence).
