Page:Collected Physical Papers.djvu/329

Graduation of the Intensity of Stimulus

The important conditions for securing quantitative results are (1) the maintenance of uniform intensity of stimulation in certain experiments and (2) the graduated increase of intensity in others.

Stimulus of torsional vibration.—I find that torsional vibration affords a very effective method of stimulation. The plant-stalk may be fixed at one end, the other end being held in a tube provided with clamping jaws, A rapid torsional to-and-fro vibration can now be imparted to the stalk by means of the handle H. The amplitude of vibration, which determines the intensity of stimulus, can be accurately measured by means of a graduated circle. The plant chamber and the vibrational stimulator is shown in fig. 85.

Fig. 85. The plant chamber. Amplitude of vibration which determines the intensity of stimulus is measured by the graduated circle seen to the right. The glass chamber is air-tight. The plant is clamped in the middle by C which acts as a block. Temperature is regulated by the electric heating-coil R. For experiments on anæsthetics, vapour of chloroform is blown in through the side tube.