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
