428 PSYCHOLOGY. 



other kind of sensation altogether, — one may, for example, in experi' 

 menting with sound, register a flash of light, produced either by 

 accident or design. We cannot well explain these results otherwise 

 than by assuming that the strain of the attention towards the impres- 

 sion we expect coexists with a preparatory innervation of the motor 

 centre for tlie reaction, which innervation the slightest shock then 

 suffices to turn into an actual discharge. This shock may be given by 

 any chance impression, even by one to which we never intended to re- 

 spond. When the preparatory innervation has once reached this pitch 

 of intensity, the time that intervenes between the stimulus and the 

 contraction of the muscles which react, may become vanishingly 

 small."* 



" The perception of an impression is facilitated when the impres- 

 sion is preceded by a warning which announces beforehand that it is 

 about to occur; This case is realized whenever several stimuli follow 

 each other at equal intervals, — when, e.g. we note pendulum movements 

 by the eye, or pendulum-strokes by the ear. Each single stroke forms 

 here the signal for the next, which is thus met by a fully prepared at- 

 tention. The same thing happens when the stimulus to be perceived is 

 preceded, at a certain interval, by a single warning : the time is 

 always notably shortened. ... I have made comparative observa- 

 tions on reaction-time with and without a warning signal. The im- 

 pression to be reacted on was the sound made by the dropping of a 

 ball on the board of the ' drop apparatus.' .... In a first series no 

 warning preceded the stroke of the ball; in the second, the noise made 

 by the apparatus in liberating the ball served as a signal. . . . Here 

 are the averages of two series of such experiments : 



Height of Fall. Average. Mean Error. No. of Expts. 



^. j No warning 0.253 0.051 13 



^5 cm. -j -vVarning 0.076 0.060 17 



. (No warning 0.266 0.036 14 



^ ^™- ] Warning 0.175 0.035 17 



" . . . In a long series of experiments, (the interval between warn- 

 ing and stimulus remaining the same) the reaction-time grows less and 

 Jess, and it is possible occasionally to reduce it to a vanishing quantity 

 (a few thousandths of a second), to zero, or even to a negative value. f 

 .... The only ground that we can assign for this phenomenon is the 

 pj-epa ration (vorbereitende Spanmmg) of the attention. It is easy to 

 understand that the reaction-time should be shortened by this means; 

 but that it should sometimes sink to zero and even assume negative 

 values, may appear surprising. Nevertheless this latter case is also 

 explained by what happens in the simple reaction-time experiments " 

 just referred to, in which, " when the strain of the attention has reached 



e -^ 



* Physiol. Psych., 2d ed. ii. 226. 



f By a negative value of the reaction-time Wundt means the case of the 

 reactive movement occurring before the stimulus. 



