14 ROBERT M. YERKES AND JOHN B. WATSON 



radiation meters." ' It is extremely sensitive, as compared 

 with the radiomicrometer, and it has the great advantage of 

 being serviceable where only a moderate degree of stability is 

 possible. In an adaptation to our needs made by Professor 

 A. H. Pfund, the selenium cell is now being tested by Professor 

 Watson. In view of the results obtained, the apparatus promises 

 well for measurements of both light and color stimuli. It provides 

 a convenient means of equating any two light, size, or form 

 stimuli with respect to intensity of light. The measurements 

 may be made quickly as well as accurately. Subsequently, 

 if it prove desirable, determinations of the photometric values 

 of the stimuli may be made. In as much as it promises to 

 be especially valuable in connection with studies of color vision, 

 further description will be reserved for our section on the meas- 

 urement of chromatic stimuli. 



We recommend that photometric measurements of light 

 stimuli be supplemented by radiometric measurements, obtained 

 by the use of some form of radiomicrometer, selenium cell, or 

 other heat meter. 



3. Application of stimulus to animal 



a. Common methods. — There are almost as many methods of 

 applying achromatic stimuli as there are investigators. Many 

 of the methods in common use are excellent for qualitative 

 investigations, but few, if any, are satisfactory for quantitative 

 work. 



The type of method - most in evidence depends upon the 

 reflection of varying amounts of light (natural or artificial) 

 from the surfaces of paper, wood, glass or metal. White, grey, 

 and black papers and cardboards are extensively used as visual 

 testing materials. They are unsatisfactory in that it is extremely 

 difficult to describe accurately the conditions of stimulation 

 and almost as difficult to control or modify them. 



b. Method recommended. — The method which it is our purpose 

 to describe in Section III as a standard procedure for testing 



' Coblentz. Loc. cit., p. 454. 



- Graber, V. Grundlinien zur Erforschung des Helligskeits und Farbensinns der 

 Tiere. Prag und Leipzig, 1884. 



Small, W. S. An experimental study of the mental processes of the rat. Amer. 

 Jour. Psychol., 1899, vol. 11, p. 80, p. 133; 1900, vol. 12, p. 20G. 



Kinnaman, A. J. The mental life of two Macacus rhesus monkeys in cap- 

 tivity. Amer. Jour. Psychol., 1902, vol. 13, pp. 98-148, 173-218. 



