430 PSYCHOLOGY. 



append two examples. ... In Series I a strong and a weak sound 

 alternated regularly, so that the intensity was each time known in ad- 

 vance. In II they came irregularly. 



I. Regular Alternation. 



Average Time. Average Error. No. of Ezpts. 



Strong sound 0.116" 0.010" 18 



Weaksouud 0.127" 0.012" 9 



II. Irregular Alternation. 



Strong sound 0.189' 0.038" 9 



Weaksound 0.298" 0.076" 15 



"Still greater is the increase of the time when, unexpectedly into a 

 series of strong impressions, a weak one is interpolated, or vice versa. 

 In this way I have seen the time of reaction upon a sound so weak as 

 to be barely perceived ri.se to 0.4" or 0.5", and for a strong sound to 

 25". It is also matter of general experience that a stimulus expected in 

 a general way, but for whose intensity attention cannot be adapted in 

 advance, demands a longer reaction-time. In such cases . . . the 

 reason for the difference can only lie in the fact that wherever a prepa- 

 rr,.tion of the attention is impossible, the time of both perception and 

 volition is prolonged. Perhaps also the conspicuously large reaction- 

 times which are got witli stimuli so faint as to be just perceptible may 

 be explained by the attention tending always to adapt itself for some- 

 thing more than this minimal amount of stimulus, so that a state ensues 

 similar to that in the case of unexpected stimuli. . . . Still 

 more than by previously unknown stimuli is the reaction-time 

 prolonged by wholly unexpected impressions. This is sometimes acci- 

 dentally brought about, when the observer's attention, instead of being 

 concentrated on the coming signal, is dispersed. It can be realized 

 purposely by suddenly thrusting into a long series of equidistant 

 stimuli a much shorter interval which the ob.server does not expect. 

 The mental effect here is like that of being startled ; — often the startling 

 is outwardly visible. The time of reaction may then easily be length- 

 ened to one quarter of a second with strong signals, or with weak ones 

 to a half-second. Slighter, but still very noticeable, is the retardation 

 when the experiment is so arranged that the observer, ignorant whether 

 the stimulus is to be an impression of light, sound, or touch, cannot 

 keep his attention turned to any particular .sense-organ in advance. 

 One notices then at the same time a peculiar unrest, as the feeling of 

 strain which accompanies the attention keeps vacillating between the 

 several senses. 



"Complications of another sort arise when what is registered is an 

 impression anticipated both in point of quality and strength, but ac- 

 companied by other stimuli which make the concentration of the atten- 

 tion difficult. The reaction-time is here always more or less prolonged. 

 The simplest case of the sort is where a momentary impression is regis- 

 tered in the midst of another, and continuous, sensorial-stimulation of 

 considerable strength. The continuous .stimulus may belong to the 



