It has been found, for example, that over-assiduous efforts at filtering and purifying the air used for ventilation so as to remove all noxious chemical and bacterial ingredients defeat their own end. Such air, although to our artificial tests absolutely clean and pure, seems to the sense of smell to lack freshness. And the nose is right. The tests are wrong. For sojourn in such an atmosphere induces lassitude and torpor of mind, as members of the Houses of Parliament, where this method has been tried, know to their cost—and ours.
But albeit so highly sensitive to minute traces, the sense occasionally fails to perceive a highly concentrated odour.
For example, every one is aware that a bunch of violets which is filling a room with its fragrance seems when held to the nose to have no smell at all, or at the most to have but a vague, indefinable sort of odour.
The effect, as a matter of fact, varies with the perfume employed. Some, like violets, have no smell at all. Others give a different smell when concentrated from what they give when dilute. Muskone, for one, the essential constituent of musk, has an odour of pines when concentrated; and storax, a delightful perfume when dilute, is disagreeable when too powerful, and so on.
It is to be noted that the disagreeable character of these last is not due to the mental “cloying” or “sickening” of excessive sweetness; it is a definite odour. Nor is the anosmia for concentrated violets due to the exhaustion of the sense.
Heyninx, comparing, as we shall see, olfaction with vision, believes the indefinite odour of concentrated violets to be akin to the absence of colour in white light. But this explanation seems to me to be improbable, since the effect is due not to the combination of a number of odours, as white light is the combination of all the colours of the spectrum, but to the overpowering influence of a single odour.
Indeed, none of the other senses shows the same phenomenon. If we happen to catch a momentary glimpse of the noonday sun, we plainly see a disc of intense light (it is pale blue in colour to my eye), surrounded by a fiery halo, before it blinds us. In the same way, when a gun is fired close to the ear, we hear the sound before we are deafened by it.
It is for such reasons that perfumers never sniff at a bottle of scent; they take a little, rub it on the back of the hand, and then wait until the spirit has evaporated before they proceed to smell it.
The exquisite delicacy of the sense might lead us to suppose that the olfactory organ must be quick at responding to its proper stimulus. But such is not the case. It is, on the other hand, relatively “slow in the uptake.”
Gleg has estimated that the reaction time for auditory sensation is from 0·12 to 0·15 of a second, whereas the reaction time for smell is as much as 0·5 of a second, only one sensory stimulus being slower, that of pain, namely, which occupies 0·9 of a second.