Summing the whole matter up, then, we may 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). Again, there is a harmony existing between certain of the manufacturers’ primitive odours; “they go well together,” and are employed for that reason in the art of perfumery. This resembles the harmony existing in another class of undulations, the sound waves. On the other hand, just as one sound may silence another by the clashing of their waves, so one odour may “kill” or neutralise another odour (iodoform and coffee, _e.g._). There are several other minor phenomena which are in agreement with this theory. They need not detain us. We turn now to the criticism of the undulatory theory of odour. First of all, we shall dispose of an objection which, at first sight, has a very serious aspect. It may seem difficult to understand how vibrations which appear to us when of a certain rate to be light should when they are of another rate become to us smell. How can one and the same physical condition produce sensations so different? The same difference, however, is encountered when we pass to the rays at the other end of the spectrum, the reds and infra-reds. On one side of the dividing line we only perceive these as heat; on the other side they also become light. Obviously, the difference can only be due to the different character of the sensory end-organ, the receptor of these vibrations. As Head says: “Each peripheral end-organ is a specific resonator attuned to some particular kind of physical vibration”—reminding us not only of soundresonators, but also of wireless receivers, which are “tuned” or accommodated to particular wave-lengths.