THE APPLICATION OF QUANTITATIVE STIMULUS 33 



of the graduated circle and movable stops : and, second, the 

 vibration period must be uniform. This last condition is 

 effected by an arrangement shown in fig. 23. The torsion- 

 head is kept tense by means of a stretched spiral spring, s, 

 made of steel. From this torsion-head there projects an 

 elastic brass piece, B. R is a striker which can be made to 

 give a quick stroke to B, by the rotation of the handle. A 

 quick to-and-fro vibration is thus produced, by the blow 

 given to B, acting against the tension of the antagonistic 

 spring S. The amplitude of the angular vibration is at the 



FIG. 23. Spring Attachment for obtaining Vibration of Uniform Rapidity 



same time predetermined by means of the stops P and Q. 

 The arrangements described are as used in ordinary work. 

 But for certain experiments on differential excitability, a 

 second striker, R', may be attached to the other end of the 

 apparatus, and by this means the opposite contacts in con- 

 nection with E and E' may be excited simultaneously. 



In order to obtain responses of great amplitude, it is now 

 necessary to increase the amplitude of vibration. But this 

 may give rise to fatigue. By way of avoiding this, therefore, 

 it is still possible to obtain enlarged response by the additive 

 effect of repeated feeble stimuli. In the electrical response 

 of plants a sub- minimal stimulus, singly ineffective, is found 



D 



