i9o8] OF SUBLIMINALLY COLORED STIMULI. 383 



of zonal limits; (5) unnoticed variations, physical, physiological or 

 psychological, in the conditions of observation during a series.'^ 



We are well aware that negative experiments are logically in- 

 conclusive.-'^ The fact that we have failed to find the colored after- 

 image does not prove that this after-image is non-existent. We 

 have, however, attempted a positive explanation : for Tschermak's 

 result, in terms of prepossession and inaccurate method ; for Miss 

 Fernald's result, in terms (predominantly) of chromatic adaptation. 

 Further experimentation by other observers must show whether our 

 hypotheses are correct. 



We are aware, also, that the charge of prepossession is double- 

 edged, and that we may ourselves be accused of an initial bias. We 

 freely confess that we, as well as Professor Baird, approached the 

 peripheral experiments in a sceptical attitude of mind. On the 

 other side, we may point out that the scepticism was positively based 

 upon the results of Baird's Carnegie Institution research, and that 

 the student-observers at the University of Illinois knew nothing of 

 the question at issue. 



In the case of Tschermak's observation, however, our initial bias 

 was positive ; we were surprised at the uniformly negative character 

 of our results with the Marbe mixer. Tschermak's position seemed 

 to accord well with current visual theory. Moreover, we knew 

 that a contrast-color may be more saturated, may appear more 

 " real," than the inducing, objective color. We knew that Hey- 

 mans, in his experiments on " psychische Hemmung," had some- 

 times seen the contrast-color while the inducing color was still un- 

 perceived.-* We knew of Helmholtz' statement, " dass die gesat- 



^ In a letter to T Miss Fernald remarks : " You will see that colored after- 

 images were seen in less than one third of the total number of cases in which 

 the stimulus-color was not seen." In a communication made to Professor 

 Baird, she estimates, roughly, that the phenomenon appeared in about five 

 per cent, of her exposures upon the peripheral retina. This sporadic and 

 fortuitous character of the after-images suggests that they are the product 

 of some variable condition which has not been taken account of in the 

 investigations. 



"'J. S. Mill, "A System of Logic," 1884, 515; W. S. Jevons, "The Prin- 

 ciples of Science," 1900, 434. 



^* G. Heymans, Untersuchungen iiber psychische Hemmung, i. Zeits. f. 

 Psychol, u. Physiol, d. Sinnesorgane, XXL, 1899, 328. "Wo mit weissen 

 Sectorenscheiben experimentirt wurde, karri es ofters vor, dass ehe noch der 

 Ring die Farbe des Papierstiickes erkennen liess, sich im Hintergrunde schon 

 die Contrastfarbe bemerklich machte." 



