SIZE AND FORM PERCEPTION 97 



two series. These two series cannot be called typical of my 

 experiments, yet, on the whole, they fairly represent the prin- 

 ciples of the chick's mode of discrimination. 



On the basis of choices, behavior, and time, it thus appears 

 that the chick's limit in discrimination from a standard o 28+ 

 lies somewhere between o 15+ and o 19+. To express the 

 relation in terms of centimeters instead of square centimeters, 

 the size in diameter of the variable is above 4.5 but below 5, 

 that of the standard is 6. The threshold of difference with a 

 standard 6, therefore, is one-fourth to one-sixth. 



2. Technique in experiments on size 

 After working only a short time on size perception I was 

 convinced that the chick which was being trained should be 

 aided by having the differences between the two visual stimuli 

 emphasized by a combination of light factors which would aid 

 in the earlier discrimination. Two stimulus areas, differing 

 from each other with respect to one quality only, present to 

 the animal a problem which is quite unnatural. Under natural 

 conditions it relies upon combinations of many visual factors. 

 The perception of an isolated visual quality is a human problem, 

 not an animal problem. 



After a little preliminary sparring, therefore, the chick which 

 was being trained was first presented with two stimuli differing 

 from each other with respect to size, form, and brightness. The 

 stimulus to be chosen was a triangle larger and brighter than a 

 circle. On account of these conditions the right stimulus ap- 

 peared in the lighter compartment. The chick was thus trained 

 to choose (1) the lighter compartment in which the stimulus 

 area was (2) larger, (3) triangular, and (4) brighter. The process 

 now was to remove, if possible, so gradually that the chick was 

 aware of no changes, all of the inequalities between the two 

 stimuli except size difference. This was not difficult to do. 

 As is indicated in table 3, undesired factors were removed before 

 the 14th series, and the single record that is given in the table 

 for o 28+ — o 7+ discrimination is the first one in which the con- 

 ditions were so arranged that the chick could not possibly choose 

 on the basis of any factor other than size. As the table also 

 suggests this was the first and only o 28+ — o 7+ series of this 

 sort; the next deals with o 28+ — o 12+ discrimination. 



