PHYSIOLOGY OF OLFACTION 51 



that surface by an appropriate movement of the inner 

 tube will produce a stimulus of two olfacties and so 

 forth. Thus a graded series of measured olfactory stim- 

 uli can easily be obtained. Further, by using outer 

 tubes carrying different odorous substances, various com- 

 parisons can be instituted as measured in olfacties. 

 Moreover, a double olfactometer (Fig. 14) may be easily 



14. Double olfaotometer. After Zwaardemaker, 1895, Fig. 15. 



devised in that two single olf actometers may be combined 

 so that one current carrying an odorous material of a 

 given concentration may be introduced into one nostril 

 and another carrying a second odorous substance of 

 known concentration can be introduced into the other 

 nostril, or both currents may be united and the odorous 

 mixture thus produced can be let into one nostril. Thus 

 a variety of comparisons may be easily made. 



Van Dam (1917b) has modified Zwaardemaker 's ap- 

 paratus by applying the odorous material in the form of 



