DISCRIMINATIVE RESPONSES TO VISUAL STIMULI 491 



of a floor-lamp or desk-lamp, placed so as to give a comfortable 

 brightness over a large central area, leaving the extreme periph- 

 eral field fairly dark. 



In the present experiment, if the area of the test-object had 

 been sufficiently increased, a value would have been found at 

 which the stimulus-surface, instead of the surroundings, deter- 

 mined the state of adaptation. In such a situation, the influence 

 of the surroundings might have been nullified or even reversed. 

 The results as reported are therefore to be regarded as holding 

 under the special conditions of the experiment, and not neces- 

 sarily holding otherwise. 



The plan of experimentation required next an application of 

 the method to a wide range of relative brightnesses of field and 

 surroundings, to find the optimal relation between them with 

 the original area of the stimulus; and next, a variation of the 

 area of the stimulus to ascertain whether the effects already 

 found are general or not. The work, however, was interrupted 

 by the national emergency of 1917; and not long afterwards, the 

 experimenter's relations with the laboratory were terminated 

 upon his entering the military service. The problem is therefore 

 abandoned, with the following comment on method : 



Threshold-determinations as a measure of the influence of 

 external conditions are meaningless unless a time-limit is placed 

 on the period of observation, or unless the time required for 

 observation is considered. If the observer maintains as in an 

 ideal case a constant criterion, the difference between the 

 threshold readings made under different external conditions need 

 give no indication of the relative facility of observation. Under 

 a relatively difficult condition the better observer tends to com- 

 pensate by making a greater effort, and extending the period 

 of observation; the less reliable observer, if he fails, may fail by 

 shifting his criterion or by declining to make the necessary 

 increase of effort. 



A special form of threshold-determination consists in requiring 

 the observer to keep an instrument continuously in threshold 

 adjustment for a considerable period of time. This method is 

 unsatisfactory, as the subject's attention is variable, and the 



