“This consists of two tubes that slide one within the other, and so shaped that one end of the inner tube may be applied to the nostril. The odorous material is carried on the inner surface of the outer tube. When the inner tube, which is graduated, is slipped into the outer one so as to cover completely its inner face, and air is drawn into the nostril through the tube, the odorous surface, being covered, gives out no particles, and no odour is perceived. By adjusting the inner tube in relation to the outer one, whereby more or less of the oderous surface is exposed, a point can be found where minimum stimulation occurs. The amount of odorous substance delivered under these circumstances to the air current has been designated by Zwaardemaker as an olfactie, the unit of olfactory stimulation. Having determined for a given substance the area necessary for the delivery of one olfactic, doubling that surface by an appropriatc movement of the inner tube will produce a stimulus of two olfacties, and so forth. Thus a graded series of measured olfactory stimuli can easily be obtained. Further, by using outer tubes carrying ditferent odorous substances various comparisons can be instituted as measured in olfacties”(Parker).
Instruments more elaborate and of greater accuracy have, as a matter of fact, been devised and used, but they need not detain us.
The results obtained by these and other methods of determining the minimum stimulus of olfaction are certainly astonishing, and reveal as nothing else can the delicate acuteness of the sense.
Fischer and Penzoldt found that they could plainly smell one milligram of chlorphenol evaporated in a room of 230 cubic metres capacity. This is equivalent to 1⁄230,000,000 of a milligram to
