DISCRIMINATIVE RESPONSES TO VISUAL STIMULI 489 



For all the subjects the dispersion of the results is greater 

 when the surroundings are 2.25 times as bright as the stimulus, 

 than when they are 0.75 times as bright as the stimulus. The 

 magnitude of the difference varies between 11 per cent and 44 

 per cent for the different subjects, and in seven sets out of eight 

 it is 8 to 15 times its probable error. 



For one subject, M, the dispersion of the results is about 

 twice as great when the surroundings are dark as when they 

 are excessively bright; and the differences are large with respect 

 to their probable errors. For the other three subjects the differ- 

 ences are small and their significance is doubtful. 



Three of the four subjects made a much smaller number of 

 incorrect or anticipatory reactions when the surroundings were 

 75 per cent of the brightness of the stimulus, than when they 

 were dark or excessively bright. Subject C is an exception, 

 exhibiting no significant difference. She made few errors under 

 any condition after the first few days of practice. 



DISCUSSION 



The results show that while individual differences exist among 

 the subjects used, visual performance was more accurate and 

 certain when the surroundings were of the same order of bright- 

 ness as the stimulus than when they were dark or when they 

 were considerably though not excessively brighter. If the latent 

 period of the muscular tissue involved, and the latent period 

 of the retina were added and the sum taken from the time re- 

 quired for the type of reaction given by the subjects, the small 

 absolute differences obtained would appear relatively much 

 larger than they do when added to a larger quantity. If moder- 

 ate differences in the distribution of brightnesses have in general 

 the effect of speeding and retarding visual performance to the 

 extent indicated by the results obtained under these special 

 conditions, the matter of appropriate distribution of brightnesses 

 would seem to be of greater importance than has been attributed 

 to it by illuminating engineers. So far, they have concerned 



