504 PHYSIOLOGY 



of surface exposed, and therefore can be diminished or increased by pushing the bent 

 tube further in, or by drawing it out. If the tube is pushed in so far that the smell 

 is only just perceptible the length of the tube may be measured and taken as the liminal 

 intensity of stimulus for the given substances, in its action on the olfactory nerve- 

 endings. This unit was called by the inventor of the instrument an ' olfactie.' By 

 this means it is possible to make quantitative estimations of the olfactory sense on one 

 individual and to compare them with observations made on other individuals. By 

 using two such instruments it is possible to present different smells to the two nostrils. 

 One obtains in this way combination effects which can be compared to the phenomenon 

 which we shall study later in dealing with binocular contrast. 



