DETECTION OF LATENT STIMULUS 



471 



length of time. The curve thus obtained ought instanta- 

 neously to mark the fluctuations of rate throughout the 

 period in question. This I have been able to accomplish 

 by means of a modified Method of Balance. From the 

 description of that method already given, it will be under- 

 stood that after the establishment of the average balance, if 

 the rate increases, the curve will move upwards. When, 

 after this, the rate of growth returns to the normal, the 

 balance will be re-established, and the curve become again 

 horizontal ; but if the growth at any time should fall below 

 the average, the curve will 

 descend. In this way pe- 

 riodic fluctuations in the 

 rate of growth may be 

 recorded. 



For the purposes of the 

 modified method of record, 

 the compensating arrange- 

 ment used for balance has 

 to be somewhat altered. 

 In recording these long 

 periodic changes, we have 

 fluctuations of larger am- 

 plitude than those of au- 

 tonomous pulsation. The 

 spot of light which is 

 thrown from the mirror of 

 the experimental Optic 

 Lever upon the second, or 



compensating, mirror is 



Fig. 188. Hydrometric Apparatus for 



Recording Continuous Variation of 



Rate of Growth 

 F, float, supporting the plant, which is 



attached by thread to optic lever, L. 



w, wheel, for adjustment of balancing 



overflow. 



thus apt to fall outside 

 the range of the latter. In 



order to overcome this difficulty I mount the plant on the 

 float itself, and adjust the outflow of water from the 

 cylinder, so that the upward growth of the plant is, at a 

 given moment, exactly compensated by the descent of the 

 float supporting it. Deviations above or below the balanced 

 rate of growth may then be followed with a recording pen, 



