Il6 Journal of Comparative Neurology and Psychology. 



Most of these experiments were performed in an apparatus which 

 I have called a "light grader." I have given a detailed descrip- 

 tion of this apparatus in another paper (Mast 'o6, p. 364). The 



Fig. 5. 



Fig. 5. A vertical section of the light grader. The lens (a) which is a segment of a cylinder has 

 its longitudinal axis lying in the plane of the section; h, stage; c, Nernst glower; d, non-reflecting back- 

 ground; e, mirror; /, light rays; g, opaque screens. Distance from glower of lamp to stage, one meter. 



Fig. 6. Stereographic view of light, lens, and image; a, lens; b, field of light produced by the image 

 of the glower (c); d, opaque screen which lies flat on lens and contains a triangular opening which 

 causes a gradation in the light intensity of the field (fc). 



important features of the apparatus will be readily understood, 

 however, by referring to the accompanying figure. 



