METHODS OF STUDYING VISION IN ANIMALS 11 



2. Measurements of stimulus 



Measurement of the photic stimuli employed in a quantitative 

 study of vision is essential. Only on the basis of exact state- 

 ments concerning the values of stimuli can the conditions of 

 an observation be reproduced and the reliability of the obser- 

 vation determined. 



Two varieties of measurement are available at present for 

 students of visual reactions. They may be designated as (a) 

 the psycho-physiological, and (b) the physical. As one of the 

 most important examples of the former variety of method we 

 have photometry; as a typical example of the latter method, 

 radiometry. 



Photometry involves optical comparison of two illuminated 

 areas, one of which is the standard and the other the compared 

 area. This comparison results in a measurement of the visual 

 stimulus in terms of a conventional unit of light (candle-power, 

 hefner, carcel). The method, in all of its forms, is dependent 

 upon the visual capacity, training, and the special skill of the 

 observer who attempts to use it. For this reason, and others 

 only less important, it is usually desirable to supplement photo- 

 metric measurements of photic stimuli by measurements of 

 their value in terms of energy. Hence the pertinence of phy- 

 sical methods. 



Determination of the value of photic stimuli in terms of heat 

 units by a radiometric procedure has proved feasible. Radio- 

 metry yields a measurement which is relatively independent of 

 the visual peculiarities of the observer, and it therefore supple- 

 ments in an invaluable manner the results of photometry. 



a. Photometry of white light stimuli. — It is difficult to obtain 

 accurate measurements of the stimuli employed in investigations 

 of vision which are conducted in daylight. But whenever a 

 dark-room is used, it is possible to employ photometry profitably. 

 We shall limit our recommendations to dark-room measurements. 



The Lummer-Brodhun prism photometer,' with a standardized 



' Brief descriptions and prices of the various forms of the Lummer-Brodhun 

 photometer may be found in the " Photometrische Apparate " catalogues of Franz 

 Schmidt and Haensch, Berlin, Germany. The catalogues may be obtained, and 

 importations satisfactorily made, through Mr. James G. Biddle, 1114 Chestnut 

 Street, Philadelphia, Pa. See also Stine, W. M., " Photometrical measurements, " 

 New York, 1904. 



