NOTES 143 



soon as the appropriate form of response appeared, the experimenter would, by 

 eliminating the three factors one after another, ascertain the following important 

 facts: first, which of the three characteristics of the objects the animal naturally 

 uses, as a basis for discrimination; second, the order of importance of the three 

 factors for the purpose of discrimination; third, the ease, accuracy, and degree 

 of delicacy of discrimination by means of each of the factors. Should the organ- 

 ism depend upon brightness at first, the experimenter might gradually diminish 

 the brightness difference of the two objects until dependence upon it for discrimi- 

 nation became impossible. He might thus gradually bring the animal to attend 

 to either the size or the form of the objects, or to both. Should it depend upon 

 size, difference in form might immediately be eliminated and the experimenter 

 would then be free to proceed with his special study of the characteristics of per- 

 ception of size in the organism. 



In this experimental procedure there are, obviously enough, two important 

 points: first, the use of conditions at the beginning of an investigation which are 

 readily discriminated by the animal and which therefore permit of the acquisition 

 of a habit quickly and without risk of discouragement; second, the directing of 

 attention from the more readily perceived to the less readily perceived factors or 

 characteristics of the objects through the gradual elimination of the more impor- 

 tant factors; third, the simplification of conditions until, finally, discrimination is 

 made to depend upon a single sensory factor. 



By employing the discrimination method in some such fashion as has been sug- 

 gested, the experimenter may not only obtain definite knowledge of the relative 

 importance of different visual characteristics of objects, but he may also succeed 

 in getting his animals to distinguish objects by means of a factor which other- 

 wise could not be made use of. The secret of success is the gradual progress from 

 the more important to the less important sensory factors of the situation. Several 

 of us have made the grave mistake of demanding of our animals attention to objects 

 which, although markedly unlike for us, were probably very much alike for them, 

 or we have made the no less serious mistake of beginning with objects which differed 

 in a number of important respects for the animals, and as soon as discrimination 

 appeared we have eliminated factors and thus abruptly increased the difficultness 

 of discrimination. Naturally, we have experienced difficulties in getting our 

 animals to attend to the objects and also in preventing discouragement and refusal 

 to attempt to discriminate. 



There is further the important possibility of saving time and of obtaining posi- 

 tive results where they otherwise would not be likely to appear by the introduc- 

 tion of still other variations in the objects. 



Professor Watson, in using the discrimination method for the study of color 

 vi-ion, lias employed the device of a moving stimulus in contrast with a stationary 

 stimulus. Thus, for example, a red stimulus area which is constant in position, 

 size, shape, and brightness is presented beside a green stimulus area of like size, 

 shape, and brightness, but appearing intermittently. At first, the intermittent 

 stimulus may be interrupted at intervals of one second. This serves to attract, 

 the animal's attention to the visual areas. Gradually the rate of interruption of 

 the one stimulus may be increased until flicker entirely disappears and the animal's 

 attention may then continue to be directed to areas whose only difference is one 

 of color. 



