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.
Thus, if red rays encounter certain tactile end-organs in the skin, they are perceived by the mind as heat, and if they pass into the eye and stimulate the retina, they are perceived as red light. In other words, in whatsoever manner an end-organ is stimulated, it only induces its own particular sensation.
How it comes about that the various end-organs induce such different sensations is not yet known.
The ultra-violet theory of olfaction, however, has to run the gauntlet of much more serious criticism than the difficulty we have just disposed of.