VISUAL PERCEPTION OF THE CHICK 17 



sence of an orienting background or environment (pattern), as 

 in the dark room studies, there may yet be a fundamental dif- 

 ference. This difference has been arbitrarily called "shape." 

 An illustration of form similarities having shape differences is 

 furnished in the experimental setting of Lashley's study. He 

 used two identical forms in that both were rectangles 2 mm. by 

 60 mm. In his use, however, they differ in this respect: one is 

 extended laterally thirty times as far as its vertical extension; 

 the other is extended vertically thirty times longer than later- 

 ally. This has been defined as a difference in shape of two 

 identical forms. 



It is not to be denied that a triangle with vertex up differs 

 from a triangle with vertex down, but one can scarcely say that 

 they are different forms. They are both triangles, and more; 

 they are equilateral triangles. Obviously it is desirable to ad- 

 here as far as possible to an explanation of the difference between 

 these patternless stimuli in perceptual terms instead of resorting 

 to ideational content. It seems useful to employ a term like 

 "shape," therefore, which may differ in forms that are other- 

 wise identical. When the extended base of the triangle is so 

 placed as to stimulate the region of the retina which was for- 

 merly stimulated by the vertex of the triangle, a condition oc- 

 curs similar to that pointed out regarding Lashley's forms: the 

 forms remain identical, but the lines of maximum and minimum 

 extension become interchanged. This fact led me to conclude 

 in my paper (2: p. 110) that the apparent reactions to form are 

 the result of keen perception of size differences. It seems prob- 

 able that they are due to the perception of these so-called shape 

 differences. The inversion of the triangle causes certain partic- 

 ular size changes — vertex or point interchanged with base or 

 line — which introduces a change in shape, but no general change 

 of size since the area remains constant. Similarly, the factor of 

 triangularity remains constant and the form is unchanged. Not 

 "the perception of form," therefore, but the perception of shape 

 "is based precisely on the unequal stimulation of different parts 

 of the retina." 



Our definition, then, as separate from the distinction between 

 forms and patterns, should distinguish between forms and 

 shapes. Referring to the retinal area stimulated, there is form 

 which is general, for example a triangle. But there is a partic- 



