570 THE POPULAR SCIENCE MONTHLY. 



There are now left undetermined only two of the factors. These 

 are the time required for the two purely mental acts : first, of per- 

 ceiving 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 will- 

 ing- 



For the purpose of learning the physiological time for events of 



difierent orders, a large number of carefully-prepared experiments 

 have been made by different investigators. Though the results ar- 

 rived 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 com- 

 parison has also been made between cases where the event was an 

 electric spark and others where the event was the passage of a lumi- 

 nous point across a line, the signal being the same in each experi- 

 ment. 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 .0*77 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 sec- 

 ond. Four investigators give .148, .1505, .180, and .182 of a second, 

 as the exact results of their experiments. It Avill 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. 



