244 PERCY W. COBB 



measurement of extremely short times of exposure, say within 

 the limits of the period of momentary fixation already established. 

 The threshold may then be worked out either in terms of time, 

 in terms of angular extent or in terms of intensity, the two other 

 factors in any case remaining constant. The classic rotating 

 disc seen by reflected light will probably have to be discarded, 

 as with it the simultaneous control of the time of exposure and 

 of general contrast and adaptation factors is not practicable. 



A further suggestion is the reduction of the extent of the 

 stimulus to that of a "physiological" point, making it the same 

 order of quantity as a star-magnitude, photometrically expres- 

 sible simply in terms of illumination upon the eye. 5 It is inter- 

 esting to speculate as to whether experimentation will show that 

 by this method the threshold may be consistently expressed, 

 without further specification as to size of stimulus field, in a 

 single numeric, having the two dimensions, flux and time. 5 

 There is some evidence 6 to make this plausible, and such relation, 

 if it can be established, will be a great help in the standardization 

 of method and in the intercomparison of results in visual thresh- 

 old work. 



6 If the brightness of the stimulus be 6, and its area s, its candle-power will 

 be 6s, and the illumination (due to the stimulus) upon the eye at distance r will 

 be 6s /r 2 . The pupillary area has not yet been taken into account. Its area, a, 

 determines the light-flux which enters the eye and goes to form the image of the 

 stimulus. This flux will be a6s /r 2 . 



6 Rivers (Schafer's Text-book of Physiology, vol. II, p. 1067) says: ". . . . 

 according to Charpentier, for a given sensation a certain amount of light is neces- 

 sary, and within the limits it matters little whether this amount is extended over 

 a larger or smaller area, or reaches the eye in a longer or shorter time." The 

 limits referred to are: as to time, 0.002 to 0.125 seconds; as to spatial extent, the 

 upper limit has been described as "corresponding approximately to the dimen- 

 sions of the fovea." (p. 1055). The quantitative statements are atrtibuted to 

 Charpentier, La lumiere et les couleurs, Paris, 1888, p. 138, and Arch, d'opht.,. 

 Paris, 1890, tome x, p. 108. 



