GRADUAL PRESSURE CHANGES. 75
stantly regarded — by which the pressure was applied to the skin, and on the other end of the beam was a small table with fixed positions for counter- weights, by which, together with the position of the car, which could be started at its full velocity at once, the amount of initial pressure was determ- ined. To minimize oscillations the counterweight was removed by means of a cam.
The mode of making observations upon the volar tip of the index finger, e. g., is as follows : The arm is rested on a comfortable support, the hand turned upward and the eyes closed. A special receptacle is made to fit the whole surface of the nail into which it is laid just under the button, which is brought down to within a millimetre of it by a screw sup- porting the other overweighted end of the beam. At a signal the counterweight is lifted by the cam, and after a fixed interval of from one to four sec- onds, during which all oscillations, if there be any, has ceased, by a turn of the key the car begins to move without noise or jar, and the differentiation begins, while the time, involving the amount of in- crease or decrease of weight, is recorded by a met- ronome till the percipient decides whether the weight is increasing or decreasing and signals to stop the apparatus, and says plus or minus accord- ingly. The wrong judgments by all observers throughout were found to be so very rare that they have been disregarded. The protocol thus gives us the point of application (commonly the tip of the left forefinger), the initial weight, the absolute amount of pressure increase or decrease per second, and the time required for a judgment. As the experimenta- tions progressed the two chief causes of variation, viz. : changing degrees of attentives and of cer- tainty, steadily diminished.
