Page:Eleanor Gamble - The Applicability of Weber's Law to Smell.pdf/26

22 shorter inhaling-tube of the same diameter and the length y be pushed for about 2 mm. into the odorous cylinder. Through the other end of this cylinder, which is usually the movable part of the instrument, let a third tube of the same diameter be pushed. By moving this third tube backward and forward, the extent of odorous surface exposed to the air is varied. Let the stimulus-limen found under these conditions be b. Then a − b will be the difference in the stimulus-limen made by the adhesion of odorous particles to a tube of the length x–y. The correction to be made for adhesion to a tube of the length x will be as much greater as x is greater than x–y. If cylinders of solid odorous s1bstances be used, this correction cannot be made, even for the stimulus-limen, since it is so exceedingly small. It is impossible, moreover, to take many determinations even of the stimulus-limen in an hour with a perfectly dry and clean tube. As for the difference-limen, it is both theoretically and practically impossible to make the adhesion-correction, for to know how much greater for sensation a given stimulus is than the liminal stimulus, one would have to know beforehamd that Weber's law applied to that particular olfactory quality, and what the exact value of $Δr/r$ for the quality was. The effect of adhesion, in the first inspiration or at least in the very first few inspirations, is to decrease the strength of the stimulus, but after the first or at most after the second or third inspiration, the effect is rather to increase the strength of the stimulus, since the odor from the matter adhering to the inhaling-tube more than compensates for the 1oss of the odor of the matter which continues to adhere.

The tube must be carefully dried after it has been washed, and the subject must be trained not to breathe back into it. Yet on a damp day, the moisture left on the inside of the tube by the inspired air is no inconsiderable source of error. Bunsen computes the possible thickness of s1ch a layer at 0.00101 mm. If a glass tube is 15 mm. long and 5 mm. wide, the area of its bore will be 23.57 qmm. This would make the weight of a layer of moisture of the thickness given by Bunsen 2.38 mg. If the odorous substance is in aqueous solution, this moisture way be left out of account, but if no moisture comes from the cylinder itself, it may vitiate the results of the experiment. Since the dampness of the air varies from day to day, this error cannot well be corrected. All that one can do is faithfully to take the barometer-readings in the hope of finding in them possible explanations of erratic judgments. The experimenter must be careful to cool the inhaling-tube after