Page:Popular Science Monthly Volume 11.djvu/589

Rh For the sense of touch there is a discrepancy in the results thus far given by different investigators. According to the best authority, it appears that the physiological time in this case is about one-seventh of a second, and less than that of sight or hearing. Some, however, make it somewhat larger than this, and place it midway between those of sight and hearing.

Having thus found the entire physiological time by many carefully-prepared experiments, and having previously obtained, as has been briefly pointed out, the value of four of its factors, the value of the remaining two can be readily obtained by subtracting. We should then have the time necessarily employed in purely mental acts of distinguishing or recognizing an event and willing the signal.

One careful investigator, after experiments in which the event was the sounding of a bell, and the signal was made by pressing with the foot, tabulates the result with the following precision:

This is only slightly more than one-fifth of a second.

The two mental operations, it will be perceived, occupy somewhat more time than is required for the transmission of the impulse to the foot, and a little more than one-half of the entire time.

In all the tests thus far given the operation of the mind is as simple as possible. The terms of the problem are reduced to their lowest forms. Upon the simplest kind of a perception, and that expected, the patient is required to exercise his volition in the simplest way. There is no necessity of making comparison between two or more sensations, or of deciding between two or more courses of action.

But in the following experiment the mental acts required were more complex. The patient, upon receiving a slight shock given to the right foot, was to give the signal by moving the right hand, and upon receiving the shock in his left foot he was to give the signal with his left hand. He was left in ignorance upon which foot the shock was to come. It was found that under these circumstances the physiological time was prolonged one-fifteenth of a second beyond the corresponding time where the patient was informed which foot was to receive the shock. In the latter case there was no need of reflection, but in the former he was obliged first to decide which foot was touched, and then to decide upon the corresponding signal. One-fifteenth of a second represented the time required for these acts of the mind.

A similar test was made for the sense of sight. For a red light