50 SMELL, TASTE, ALLIED SENSES 



itself readily to the determination of absolute measure- 

 ments but it is not so easily applied to questions involv- 

 ing the comparison of odors. For the measurement of 

 olfactory acuity, but especially for the comparison of 

 odors, Zwaardemaker invented an ingenious piece of ap- 

 paratus called an olfactometer. (Fig. 13). 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 



OlfactifS 



3 k 1 



FIG. 13. Simple rubber olfactometer. After Zwaardemaker, 1895, Fig. 14. 



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 odor is perceived. If, now, the 

 inner tube is withdrawn a certain distance so that a 

 given surface of odorous material is exposed to the cur- 

 rent of air, odorous particles escape into the current and 

 these may be sufficient in amount to call forth olf action. 

 By adjusting the inner tube in relation to the outer one 

 whereby more or less of the odorous 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 olf actie, the unit of olfactory stimu- 

 lation. Having determined for a given substance the 

 area necessary for the delivery of one olf actie, doubling 



