90 ROBERT M. YERKES AND JOHN B. WATSON 



Distance perception. — It has not been possible for us to pro- 

 vide a method of testing this factor of visual discrimination 

 with our standardized apparatus without troublesome and expen- 

 sive complications. We, therefore, have left this aspect of vision 

 for examination by means of devices which may be introduced 

 in connection with our light and color apparatus. It is our plan 

 to arrange mechanical devices by means of which the distance 

 of the two stimulus areas from the animal may be altered by 

 a few centimeters without avoidable changes in the other con- 

 ditions. The testing of apparent distance as a factor in dis- 

 crimination is especially important because of the distance- 

 effects of colors. 



Color perception. — The method of applying chromatic stimuli 

 should be determined by (a) the manner in which the stimulus 

 is obtained (reflection, transmission, or dispersion method) 

 and (b) the characteristics of the animal. The first task of 

 the investigator is to decide which of the three methods of 

 obtaining stimuli will best serve his purposes. Having selected 

 one of the three general methods, he should perfect an experi- 

 ment box to use in connection with the standard light or color 

 apparatus (the light apparatus may perfectly well be used in 

 tests for color vision with either reflected or transmitted light 

 from papers, cloths, glasses, etc.) 



Our chief recommendations concerning the use of the color 

 apparatus are these: That for high intensity work either the 

 sun or an automatic open arc lamp, and for low intensity work 

 a Nernst glower, be used as a source; that compound achro- 

 matic lenses be used for condenser, collimator, objective, and 

 single achromats for small projector; that the double slit be 

 calibrated for w^ave-lengths ; that accurately cut and calibrated 

 rotating sectors be used for the control of the intensity of the 

 tw^o beams, but that iris diaphragms be used instead, for sup- 

 plementary check tests ; that extreme pains be taken to prevent 

 dependence upon secondary criteria, such as differences in 

 illumination, differences in size, form, or apparent distance o^ 

 stimulus areas; that in addition to making accurate photo- 

 metrical and radiometrical measurements of the stimuli, the 

 experimenter vary the conditions of the tests in all convenient 

 ways in order to make certain of the kind of discrimination 

 with which he is dealing. 



