PATTERN-DISCRIMINATION IN VERTEBRATES 339 



will have a uniform surface-brightness. The angular opening 

 of D 3 is 180+n , and that of D 4 , (180 — n°.) The luminous intensity 

 of this field is therefore equal to that of the plain field, but this 

 field appears striate, the degree of contrast between the dark and 

 the bright portions depending on the value of n. Interchange 

 of D 3 and D 4 with D 1 and D 2 reverses the relative positions of 

 the plain and the striate fields. Diaphragms placed between the 

 cubes and the windows of the Yerkes box limit the visual field as 

 closely as possible to the surfaces of the cubes. 



I have not yet used this apparatus in actual tests, and some 

 questions of convenience have yet to be settled. One of these 

 is a suitable means of effecting a quick and noiseless interchange 

 of the two pairs of sectors. Another important question is the 

 rate of speed at which the sectors have to be driven. At the 

 suggestion of Director Hyde of this laboratory I have decided 

 to postpone tests on contrast-sensitivity until the threshold for 

 rate of flicker under the experimental conditions shall have been 

 ascertained for each individual subject. 



Having ascertained experimentally, however, that certain 

 animals under the experimental conditions can distinguish striae 

 narrower than those on the cubes when the contrast is sharp, 

 the writer assumes without scruple that a method of preparing 

 the stimuli which meets physical requirements for that purpose 

 should show what degree of contrast is necessary to arouse such 

 discrimination. 



