4 ROBERT M. YERKES AND JOHN B. WATSON 



I. METHODS OF INVESTIGATING LWHT VISION 



I. Sources of stimulus 



A source of stimuli for the study of light vision ' in animals 

 should fulfill four important requirements which may be des- 

 ignated as naturalness, controllability, constancy, and meas- 

 urability. 



(i) Naturalness. — The source should yield light which is 

 identical with, or closely similar to, daylight or sunHght in 

 quality. Or, in other words, stimuli which are used for the 

 investigation of the visual capacity of an animal should be 

 " familiar" or "natural" to the animal. As has been pointed 

 out by Nichols, " the normal stimulus of the eye is diffuse day- 

 light and artificial stimuli which depart x^idely from it either 

 as to intensity or quality are sure to be unsatisfactory and are 

 likely to be injurious." - We may expect natural or normal 

 reactions only when stimuli to which the animal is accustomed 

 are employecl. This is the principal reason for the use of day- 

 light and sunlight, instead of artificial light, in experiments 

 on vision. 



(2) Controllability. — The source of light should be, so far as 

 is possible, under the control of the experimenter. This re- 

 quirement applies to both quality and intensity, but it is espec- 

 ially important in connection with work on light vision that 

 the intensity of the stimulus be accurately controllable. 



(3) Constancy. — The source under given conditions should 

 yield light whose quality and intensity are constant. No known 

 source fulfills this requirement. Certain of the high temper- 

 ature electric lamps are most nearly satisfactory. 



(4) Measurability. — The light which a given source produces 

 should be readily and accurately measurable in terms of visual 

 (e. g., photometric) and energy (e. g., radiometric) units. In 

 the employment of visual, and all other stimuli, it is desirable 

 to obtain three sets of measurements: the psychological (sensation- 

 unit), the physiological {stimulus-unit), and the physical {energy- 

 unit) . 



With respect to their relations to the four requirements which 

 lights used for experiments on vision should fulfill we shall 



'Frequently termed " brightness " vision. 



^Nichols, E. L. Daylight and artificial light. Ilhuninating Engineer, 1908. 



