loo 



SCIENCE. 



[Vol. XVI. No. 394 



response; in piano playing, and the guidance of complicated 

 machinery, we see more elaborate instances of similar pro- 

 cesses. The printer distributing "pi," the post-office clerk 

 sorting the mails, are illustrations of quick forms of re-action, 

 in which the different letters of the alphabet or the different 

 addresses of the mail matter act as the stimuli, and the plac- 

 ing them in their appropriate places follows as the response. 

 In many games, such as tennis or cricket, the various ways 

 in which the ball is seen to come to the striker are the 

 stimuli, for each variation of which there is a precise and 

 complex form of response in the mode of returning the ball. 

 In military drill the various words of command are the 

 stimuli, and the actions thus induced the responses; and such 

 illustrations could be multiplied indefinitely. In all these 

 actions the time-relations are more or less definite and im- 

 portant, but a useful study of them presupposes a careful 

 and systematic analysis of the processes therein involved. 

 We recognize that certain of the above actions are more 

 complicated than others, and we must inquire in what this 

 complication consists. In the process as usually presented 

 the nature of the re-action depends upon the nature of the 

 stimulus, a variation in the one being concomitant with a 

 variation in the other. The piano player, seeing a certain 

 mark on the page, strikes a certain key on the key-board, 

 but strikes a different key if this mark be differently placed; 

 the soldier varies his movement according to the word of 

 command, and so on with most of the others. All such 

 actions involve at least three processes: (1) the recognition 

 of the sense impression, (2) the performance of the appro- 

 priate action, and (3) the association of the one with the 

 other. The recognition involves the appreciation of the 

 presence together with the appreciation of the nature of the 

 sense-impression; and the movement involves the contrac- 

 tion of muscles together with the initiation of the impulse. 

 We obtain the simplest form of re-action by limiting the 

 stimulus to a single definite one, and having one and the 

 same response irrespective of the nature of the stimulus. 

 The subject expects the stimulus the nature of which he 

 knows, and is ready to signal, by a simple movement agreed 

 upon in advance, merely that the impression has been re- 

 ceived. This we shall speak of as a "simple re-action." It 

 occurs whenever a certain sense-impression is agreed upon 

 as a signal for the execution of a simple movement. The 

 time-keeper pressing the spring of the stop-watch, or the 

 racer starting off as soon as the pistol is fired or the word 

 is given, are instances of simple re-actions. It should be 

 noted that the simplicity of the act refers primarily to the 

 subject's fore-knowledge of what is to occur; the nature of 

 the sense-impression, as of the motion, is known in advance, 

 the association between the two being in the main artificial. 

 Inasmuch as the more elaborate mental processes involve 

 those of the simple re-action, our first step must be to deter- 

 mine its elements and their time-relations. 



The Elements of a Simple Re-action. 



The several elements of a simple re-action have been 

 variously analyzed by different observers, but all recognize 

 the physiological and the specially psychological portions 

 of the process. The physiological time-elements include, 

 (a) the time for the sense-organ to respond to an impression, 

 i.e., to overcome its inertia; (6) the time for the passage of 



the impulse inward along nerves (and spinal cord), with the 

 various delays whenever the impulse enters or passes through 

 cells; (c) the return passage of the motor impulse from the 

 brain to (spinal chord and) nerve and muscle; and (d) the 

 time for the contraction of the muscle. The time thus left 

 unaccounted for is that taken up by the psychological pro- 

 cess, the transformation of the sensory into the motor im- 

 pulse, — a process taking place in the brain, but as to the 

 precise nature of which we have no definite information. 

 The separate determination of each of the physiological fac- 

 tors enables us to find approximately the duration of the 

 central process. As a sufficiently typical case we may accept 

 the estimate of Cattell, that, in re-acting to a light by press- 

 ing the key with the finger, the time needed by the impulse 

 to travel from eye to brain and from brain to spinal cord 

 and finger is about SOff;' the latent time in the muscle, dur- 

 ing which it overcomes its inertia, is judged from experi- 

 ments upon the frog to be about 5(T to 10(5"; and experiment 

 gives a value of 15 ff to 20 ff for stimulating the retina and 

 initiating the impulse. As the entire re-action occupied 

 about 150(y we conclude that in this case the physiological 

 and the psychological portions of the process occupy about 

 equal times. One may obtain a fair notion of the rate of 

 these processes by the following simple experiment. A 

 score or so of persons form a chain by joining hands, and at a 

 given signal a certain member of the group sharply presses 

 the hand of his neighbor, who in turn imparts the pressure 

 as quickly as possible to his neighbor, and so on until the 

 impression has gone the rounds. An outsider keeps the 

 time (which may he done with suflB^cient accuracy by count- 

 ing the ticks of a watch, usually fifths of a second) from the 

 moment of giving the signal to start to the moment of re- 

 ceiving the signal from the last member of the group that 

 the impression has been circulated. The entire time divided 

 by the number of persons in the group (or better, by that 

 number plus two to include the re-actions at starting and 

 stopping) gives an average simple re-action-time, which, 

 though long at first, is reduced after a little practice to a 

 sixth or a seventh of a second. On this basis one may cal- 

 culate that if a number of men, stretching out their arms 

 and grasping one another's hands, were stationed in a 

 straight line, it would take three minutes to send a message in 

 the manner just described along a mile of this human telegraph. 



(a) The inertia of sense-organs has been variously deter- 

 mined. One method measures how closely impressions may 

 follow one another without fusing. The time thus measured 

 is the minimum time during which the sense-organ may be 

 stimulated and recover sufficiently to receive a second stim- 

 ulation. This process thus includes something more than 

 the one we desire to measure, and may perhaps be regarded 

 as furnishing a maximum time of the sensory inertia. Here 

 again various circumstances influence the determinations, 

 the chief ones being the sense-organ in question and the 

 clearness and intensity of the impression. Sectors of black 

 and white upon a disc revolving in dajlight at the rate of 

 about 25 times a second fusfe into a uniform gray, making 

 the inertia of the retina under these conditions about 40(T. 

 In weak light (moonlight) the time lengthens to about 100(7. 

 The same experiment has been made with sectors of different 

 colors, with the disc stationary and the light reflected from 

 a rotating mirror, with a vibrating point of light; and, while 

 all these variations somewhat affect the result, the majority 

 of the determinations indicate a fusion at 30 to 40 impressions 



* The sign o- indicates one one-thousandtli of a second. 



