THE MOMENTARY CHARACTER OF ORDINARY 

 VISUAL STIMULI 



PERCY W. COBB 



Medical Research Laboratory, Mitchel Field, Garden City, New York 



The fact that in rather common experience some distributions 

 of brightness in the visual field are more acceptable than others 

 has never found a physiological explanation satisfactory to the 

 writer. Measurements of the foveal threshold, both for form 

 and for brightness-difference, do not show, under the various 

 experimental light conditions in question, differences at all 

 commensurate with the differences in the varying degrees of 

 comfort and discomfort expressed as the introspective findings 

 of the users of lighting systems representing similar distributions. 



The experimental results which the writer has hi mind are 

 from his own work of several years ago, 1 and consist in threshold 

 measurements made in a bright rectangular field of small extent : 

 2.64 degrees horizontally and 1.95 degrees vertically. The 

 photometric difference between the two halves of this field, 

 necessary to a correct judgment of the direction of that differ- 

 ence, was taken as the threshold. The types of light distribution 

 in the visual field were brought about by having this test field 

 viewed in surroundings (or perhaps more clearly stated, upon 

 a background) whose brightness could be independently and 

 indefinitely varied. 



Thus three typical conditions were possible, as well as all 

 gradations between: (1) the condition in which field and sur- 

 roundings were of equal brightness and which was treated as a 

 standard; (2) the condition in which the surroundings were 



1 Cobb, P. W., The Effect on Foveal Vision of Bright Surroundings, iv. Jour. 

 Exp. Psycho!., i, 1916, pp. 540-66. Table V, p. 547 contains the data referred to. 

 The preceding communications under like title are to be found in: Psychol. Rev., 

 xx, pp. 425-47, ibid., xxi, pp. 23-32 and Jour. Exp. Psychol., i, pp. 419-25. 



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