There are now left undetermined only two of the factors. These are the time required for the two purely mental acts: first, of perceiving the sensation; and, second, of willing the movement which immediately follows. These two make up the psychical time. It may be obtained by first learning the entire physiological time, and then subtracting from that the value of the four known factors, the determination of which has been given. This would leave the period required for the purely mental operations of distinguishing and willing.
For the purpose of learning the physiological time for events of different orders, a large number of carefully-prepared experiments have been made by different investigators. Though the results arrived at lack much in accuracy and completeness, they are highly interesting and instructive.
For experiments relating to sight the arrangement is usually of the following nature: The patient is seated before a screen, upon which certain letters or numbers can be thrown, or before glass globes, which can be suddenly illumined by an electric spark. Upon seeing the number, letter, or illumination, as the case may be, the patient immediately gives a signal by some slight movement of the hand or foot. Nice arrangements are provided, by which both the event and the signal register themselves, giving the time exact to the thousandth of a second.
The general result of many experiments is, that the time required for ordinary persons to distinguish an appearance and to give the signal amounts to about one-fifth of a second. Exactly speaking, in the experiments of one scientist, the time required was found to be .188 of a second. In trials made by three other investigators the results have been .200, .205, and .194 of a second respectively. None of these conclusions varies much from one-fifth of a second. A comparison has also been made between cases where the event was an electric spark and others where the event was the passage of a luminous point across a line, the signal being the same in each experiment. According to one series of trials, it was found that the time required in the former case was .200 of a second and in the latter only .077 of a second. In other words, the perception and signaling of the former occupied three times as much time as in the case of the latter.
In experiments with the sense of hearing, the arrangements are generally of the same nature. The event is usually the sound of a bell or note, or of a spoken letter or number. For this sense the physiological time has been calculated to be about one-sixth of a second. Four investigators give .148, .1505, .180, and .182 of a second, as the exact results of their experiments. It will be noticed that the physiological time for the sense of hearing is shorter than for that of sight. We hear more quickly than we see.
