EXCITABILITY DETERMINED BY INTERFERENCE 143 



the same as that on the right. If the excitability of the two 

 points had been exactly the same, equal amplitudes of vibra- 

 tion would have resulted in the equal stimulation of both. 

 But in practice the excitabilities are found to be slightly 

 different and the angle of vibration of the one must, therefore, 

 be so adjusted as to induce an excitatory effect exactly equal 

 to that of the other. 



The two striking-rods, one on the right, R, and the other 

 on the left, R', can be adjusted so that both are in the same 



FlG. 99. R, R', striking-rods for stimulation of two ends of specimen ; 

 B, elastic brass tongue projecting from torsion-head. For producing 

 phase-difference R is adjustable in azimuth. 



vertical plane, or so that one is in advance of the other. 

 The left rod is permanently fixed to the rotating axis, but 

 the right can be set at any angle that is desired, with the 

 other. When the right striking-rod is set, pointing to zero of 

 the scale, the two rods are in the same vertical plane, and the 

 rotation of the handle causes equal vibrational stimulus by 

 the two at the same moment. The excitatory reactions on 

 right and left are now, therefore, of the same phase and of 

 equal intensity, but opposed to each other. In fig. 100, a, 

 are reproduced the two separate and equal constituent 

 responses given by a specimen of stem of Amaranth. The 



