1902.] Observations on " Flicker" in Binocular Vision. 73 



Tlie spindle of the revolving cylindrical screen carried a step-pulley. 

 From this a cord ran to a step-pulley fixed on the spindle of a small 

 electromotor. The speed of running of this motor was controlled by 

 a set of coil resistances, which formed a coarse adjustment, and by a 

 fluid resistance in a trough 1 metre long, with a sliding electrode ; this 

 formed a fine adjustment. The speed of rotation of the cylindrical 

 screen was recorded by marking the completion of each revolution of 

 the spindle carrying the screen by an electro-magnetic signal writing on 

 a travelling blackened surface. On the same surface the time was 

 recorded by a writing clock marking fifths of seconds. 



The observations required an operator to manage speed of motor, 

 registration of time and revolutions, &c, and an observer who, seated 

 in a dark compartment, gave his attention to the watching of the illu- 

 minated discs. 



The shutters of the rotating cylindrical screen could be arranged so 

 that the illumination of a retinal spot in one eye could be varied in 

 time and intensity synchronously or asynchronously with that of the 

 conjugate spot of the other eye. Of various combinations examined, 

 the following may be cited as outlining the evidence obtained. The 

 discs may be referred to as disc A and disc B. 



Experiment 1. — When the disc A represents right retinal stimulation 

 alone (i.e., without left), the conjugate spot of the left eye remaining 

 dark; and when the disc B represents right retinal stimulation with 

 coincident synchronous (i.e., of synchronous phase) intermittent stimu- 

 lation of the conjugate spot of the left retina ; the frequency, duration, 

 and intensity of the light periods being alike at all the holes. 



Then, steady sensation is obtained from disc A at frequencies of 

 intermission lower than those required for giving steady sensation from 

 disc B. 



At speeds sufficient to give steady sensation at both discs, the disc B 

 does not obviously differ in brightness from the disc A. 



Experiment 2.— When disc A represents as before uniocular stimula- 

 tion only, 



And when disc B represents intermittent right retinal stimulation, 

 together with a stimulation yielding steady sensation from the con- 

 jugate field of left retina. 



And when the steady sensation from left retina corresponds with an 

 intensity of light stimulus half that of the intermittent stimulus 

 employed for the right eye, then the rate of intermission required for 

 obtaining steady sensation from disc A is higher than that required for 

 obtaining it from disc B. 



' At speeds sufficient to give steady sensation for both discs, the 

 disc B does not obviously differ in brightness from the disc A. 



Experiment 3. — The disc A representing uniocular vision as before. 



When disc B represents intermittent right retinal stimulation with, 



