ATTENTION. 431 



same sense as the stimulus to be reacted on, or to another. When it is 

 of the same sense, the retardation it causes may be partly due to the 

 distraction of the attention by it, but partly also to the fact that the 

 stimulus to be reacted on stands out less strongly than if alone, and 

 practically becomes a less intense sensation. But other factors in reality 

 are present ; for we find the reaction-time more prolonged by the con- 

 comitant stimulation when the stimulus is weak than when it is strong. 

 I made experiments in which the principal impression, or signal for re- 

 action, was a bell-stroke whose strength could be graduated by a spring 

 against the hammer with a movable counterpoise. Each set of obser- 

 vations comprised two series ; in one of which the bell-stroke was regis- 

 tered in the ordinary way, whilst in the other a toothed wheel belong- 

 ing to the chronometric apparatus made during the entire experiment a 

 steady noise against a metal spring. In one half of the latter series (A) 

 the bell-stroke was only moderately strong, so that the accompanying 

 noise diminished it considerably, without, however, making it indistin- 

 guishable. In the other half (B) the bell-sound was so loud as to be 

 heard with perfect distinctness above the noise. 



No. of 

 Mean. Maximum. Mininiim. Experiments. 



A r Without noise 0.189 0.244 0.156 21 



(Bell-stroke ■. ^yj^j^ ^^^.^^^ ^ 3^3 Q^gg q jgg jg 



moderate) \ 



B r Without noise 0.158 0.206 0.133 20 



(Bell-stroke j ^y;jj^ yyjg^ ^ 203 0.295 0.140 19 



loud) I 



"Since, in these experiments, the sound B even with noise made a 

 considerably stronger impression than the sound A without, we must 

 see in the figures a direct influence of the disturbing noise on the pro- 

 cess of reaction. This influence is freed from mixture with other factors 

 when the momentary stimulus and the concomitant disturbance appeal 

 to ditferent senses. I chose, to test this, sight and hearing. The mo- 

 mentary signal was an induction-spark leaping from one platinum point 

 to another against a dark background. The steady stimulation was the 

 noise above described. 



Spark. Mean. Maximum. Minimum. No.ofExpts. 



Without noise 0.222 0.284 0.158 20 



With noise 0.300 0.390 0.250 18 



" When one reflects that in the experiments with one and the same 

 sense the relative intensity of the signal is always depressed [which by 

 itself is a retarding condition] the amount of retardation in these last 

 observations makes it probable that the disturbing influence upon atten- 

 tion is greater when the stimuli are disparate than wJieti they belong 

 to the same sense. One does not, in fact, find it particularly hard "to 

 register immediately, when the bell rings in the midst of the noise ; but 

 when the spark is the signal one has a feeling of being coerced, as one 

 turns awav from the noise towards it. This fact is immediatelv con- 



