72 



Prof. C. S. Sherrington. 



[July 30, 



fixation in time. It has been used in this enquiry as an index for 

 noting the influence of the physiological state of a spot in one retina 

 upon that of the " identical " spot in the other retina. A second index 

 taken has been the visual " brightness " of the perceived image. 



For the observations desired, it was considered important that 

 extinction and re-illumination of the image occur pan passu in the two 

 eyes, i.e., with like speed, along a like direction in the retinal surface, 

 and at " identical spots." It seemed also important to maintain per- 

 ceptual singleness of the object seen, thus placing the " identical 

 spots " in the most favourable condition for their co-operative identity. 

 For this reason a steady and considerable degree of " convergence n 

 of the visual axes was made one condition of the observations. The 

 degree of focal accommodation of the eyes was arranged to correspond 

 with the amount of convergence. Variations in the aperture of the 

 pupil were excluded by wearing small artificial pupils. 



The illuminated object was arranged as follows. A small double 

 sheet of thick " milk " glass was illuminated by a candle-shaped, single- 

 loop, incandescent electric lamp, with frosted glass front. The lamp 

 was fed at rather above its intended voltage by accumulators unused 

 during the experiment for any other purpose than the lighting of the 

 lamp. The lamp was set in the axis of a rotating cylinder. In this 

 latter were openings of appropriate size, different according to the 

 different requirements of the observation. The lamp was not fixed to 

 the cylindrical screen. The milk glass was set close inside this screen. 

 Outside the moving cylindrical screen was a fixed cylindrical screen 

 concentric with the revolving one. In the fixed cylindrical screen four 

 circular holes were arranged so that two were centred on the same 

 horizontal line, and of the other two, one was centred as far above the 

 left-hand hole of the previous pair as the second was below the right- 

 hand member of the pair. These holes were viewed from a distance 

 such that when the line of the visual direction of the left eye passed 

 through the centre of the lower left hole it met at the axis of the 

 cylindrical lantern, the line of visual direction of the right eye passing 

 through the centre of the upper right-hand hole. A blackened vertical 

 screen cut off from the left eye all view of the right-hand holes, and 

 from the right eye all view of the left-hand holes. A pair of weak 

 prisms with base-apex line vertical sufficed to bring the images of the 

 right-hand and left-hand holes to the same levels. The images then 

 immediately fused under convergence. Cross horizontal and vertical 

 vires across the component holes served to certify binocular or 

 monocular vision to the observer. When the four holes were all 

 allowed to contribute images they were seen by the observer as two 

 small evenly-lighted discs, one vertically above the other, with a 

 delicate cross-line on each. Any one of the holes could be separately 

 screened out of vision. 



