1902.] Observations on "Flicker" in Binocular Vision. 75 



tion from disc B does not appreciably differ from that required to 

 obtain it from disc A. 



Of the two discs, at speed sufficient to yield steady sensation, disc B 

 does not obviousiy differ in brightness from disc A. 



Experiment 7. — When disc A represents the binocular combination 

 described for disk B, Experiment 1, and the disk B is as described in 

 Experiment 6, the rate of intermission required to obtain steady 

 sensation from A is higher than that required for obtaining it from B, 

 but at speeds sufficient to yield steady sensation from both discs the 

 two discs appear to be of equal brightness. 



Experiment 8. — With disc A, as in Experiment 7, and disc B, as 

 described in Experiment 6, the frequency of repetition of stimulus 

 required to yield steady sensation from disc A is slightly higher than 

 that required to obtain it from disc B, but at speeds sufficient to 

 yield steady sensation from both discs the two discs appear of equal 

 brightness. 



Experiment 9. — With disc A, representing the binocular combination 

 described for disc B in Experiment 5, and with disc B, as described in 

 Experiment 6, the frequence of repetition of stimulus required to 

 yield steady sensation at disc A is slightly lower than that required to 

 yield it at disc B ; but at speeds sufficient to yield steady sensation 

 from both discs, the two discs appear of equal brightness. 



The observations show (i) that Talbot's law, unimpeachable (oyer 

 a wide range of ordinary luminosities) for the single eye, is not ap- 

 plicable to combined binocular vision, that is, that if the two eyes 

 functioning together in binocular vision are considered as functionally 

 to combine to a single organ, Talbot's law does not hold good for 

 that organ as it does for the single eye, right or left, (ii) That increase 

 of luminosity of an intermitting image does not always necessitate 

 increase of rate of frequency to extinguish its flicker ; and conversely, 

 They also show that the " Fechner paradox " regarding binocular 

 luminosity makes itself apparent under " flicker " examination as well 

 as under " brightness " estimation. 



It seems that the physiological sum of two luminosities perceived 

 through conjugate retinal areas is of a value intermediate between 

 the individual values of the two component luminosities. 



Among experimental difficulties incident to the experiments may be 

 mentioned the increased perception of flicker under paracentral as com 

 pared with central locus of stimulus on the retina, as noted by Exner 

 and by Charpentier. Interesting experimental difficulties were also 

 occasioned by the reciprocal and often antagonistic influences exerted 

 by one retina upon another in ways studied and described recently by 

 Dr. W. McDougall.* 



It was further observed that binocular colour mixture did not seem 

 * ' Mind/ 1901. 



