iv THE SENSE OF SMELL 187 



one nozzle of the olfactometer into each nostril, or both odours 

 could be made to act on one nostril alone by uniting the two 

 olfactometers by a T-junction. 



The liminal value, or minimal perceptible intensity of each of 

 the odours to be experimented with, is first determined. This 

 estimation has to be made for each nostril separately, since it is 

 rare to find both equally sensitive. The minimal amount of each 

 odour perceptible to each nasal fossa is the olfactie, and corresponds 

 to a certain length (measured in centimetres) of exposure of the 

 olfactometer tube. When this has been determined it is easy to 

 vary the intensity of the olfactory stimuli in measurable quantities 

 (of 1, 2, 3 . . . olfacties), and to change the relative intensity of 

 the two odours in the two olfactometers. 



By this method Zwaardemaker established that full com- 

 pensation is obtained when the nostrils are separately stimulated 

 with the following pairs of odorous substances in the proportions 

 indicated as follows : 



In centimetres of the 



olfactometer. In olfacties. 



Cedar wood and rubber . . . . 5-5 : 10 2-5 : 14 



Benzoin and rubber 3-5 : 10 3-5 : 10 



Paraffin and rubber 8-5 : 10 8-5 : 14 



Kubber and wax 10 : 7 14 : 28 



Eubber and balsam of Tolu . . . 10 : 7 14 : 70 



Wax and balsam of Tolu . . . . 10 : 9 40 : 90 



Paraffin and wax 10 : 5 10 : 20 



If the relative intensity of any pair of these substances is 

 altered, either the strongest smell alone is perceived, or there is a 

 conflict between the two sensations, or only a very weak and 

 indefinite sensation, or lastly, a disappearance of all sensation 

 when there is perfect neutralisation. According to Zwaardemaker 

 there is never, even with very strong odours, a mixed sensation, 

 i.e. a psychical combination of the two olfactory sensations, tending 

 to a reinforcement or sensible qualitative alteration in the percep- 

 tion of one or the other odorous substance. 



One of the most interesting experiments that can be made with 

 Zwaardemaker's double olfactometer consists in filling the one 

 with acetic acid (2 per cent), the other with ammonia (1 per cent). 

 On leading the two odorous substances separately to the two 

 nostrils, a smell of ammonia or of acetic acid is obtained, 

 according as the one or other cylinder is the more exposed. The 

 two smells are never simultaneously perceptible. It is, however, 

 possible to find such a relation of intensity that neither of the two 

 odours prevails over the other, or there is at most a weak smell of 

 one or the other. Lastly, it is possible to discover such propor- 

 tions that on sniffing with the two nostrils no olfactory sensation 

 results, even when the two stimuli are so strong that either, 

 separately, would arouse an intense sensation. 



