30 ROBERT M. YERKES AND JOHN B. WATSON 



Some are especially valuable for low intensities of light ; others 

 for high intensities. 



As sources which are especially strong in certain portions of 

 the spectrum we recommend: 



For red, orange, and yellow: Sun (low); mercury arc; amyl-- 

 acetate lamp; sodium flame; combustion gas lamps; oil lamps. 



For green: Mercury arc; welsbach. 



For blue and violet : Skylight ; enclosed carbon arc ; mercury arc. . 



2. Methods of obtaining and applying stimulus 



Of the many methods by which chromatic stimuli may be 

 obtained for experiments on vision we shall discuss the values 

 of only those which seem to us practicable in the present state 

 of our knowledge. For convenience of description we have 

 classified the methods under the three rubrics of reflection^ 

 transmission, and dispersion. There are in reality, for our pur- 

 poses, two physical phenomena which yield colored light: selec 

 tive absorption, and the resolution of white light. The former 

 gives the phenomena of object-color in nature ; the latter exhibits 

 itself in the rainbow, and in various ways through refraction 

 and interference. In the case of selective absorption, the chro-- 

 matic stimulus may come to the eye as reflected light (from 

 the absorbing surface) or as transmitted Hght, if the medium 

 be partially transparent. The phenomenon of selective absorp-- 

 tion appears, in its two forms, in colored papers and colored 

 glasses : the former yield chromatic stimuli by selective absorption 

 and reflection ; the latter by selective absorption and transmission. 



We shall briefly consider, in turn, each of the three groups of 

 methods. 



a. The re-flection method. — Here we class substances which, 

 because of their capacity for absorption, reflect only a definitely 

 limited range of wave-lengths. Chief among them, for our special 

 purposes, are colored papers, colored cloths, and oil pigments 

 on opaque substrata. 



Colored papers. — These, in various forms, have long been 

 used, and still are extensively employed in the study of human 

 color vision. The most pertinent description of them would seem 

 to be an enumeration of their merits and defects. 



Merits: AvailabiHty in many colors and saturations (hues, 

 tints, chromas);' cheapness, and convenience of handling;. 



1 Titchener, E. B. A text-book of psychology. New York, 1910, p. 54. 



