VISION 379 



lower limit of their sensitiveness and the higher degree of 

 brightness, at which they are superseded by cones ; but they 

 afford no information regarding colour. Their monochrome is 

 interpreted by the mind as a bluish grey, apparently because, 

 since they are insensitive to red rays, the sensations of which 

 they are the source are associated with the blue end of the 

 spectrum. When the cones are stimulated very slightly, the 

 reinforcing grey of the rods enables us to distinguish all other 

 colours, save red, which appears black. In bright light 

 the rods are in a permanent state of exhaustion ; they do not 

 contribute to vision. Rods respond to stimulation more 

 slowly than cones. This fact enables us, by a very pretty 

 experiment, to distinguish the two kinds of vision. A disc 

 of green paper about the size of a threepenny-bit is pasted 

 on a red surface. Held at arm's length in a room lighted 

 by a single candle, the disc looks dull green when the gaze 

 is directed at it ; but if the gaze be directed 2 or 3 inches 

 to one side of it, it appears brighter than before, but less 

 distinct and almost grey. The explanation of this is to be 

 found in the fact that at the posterior pole of the eye there 

 is a shallow cup fovea centralis which carries cones only, 

 without rods. This small depression is the area of direct vision, 

 the only spot at which we see things quite distinctly. At the 

 fovea the nuclei and nerve-cells of the retina are withdrawn 

 from in front of the cones to the margin of the cup, in order 

 that they may not interfere with the passage of light. The 

 pit and the ring round it contain some yellow pigment. Hence 

 it is usually termed the " yellow spot." When we are looking 

 straight at the green disc, it is focussed on the yellow spot. It 

 then excites a sensation of greenness ; but since this is not 

 reinforced by any rod-sensations, the green is dull. When it 

 is focussed outside the yellow spot, it stimulates rods and the 

 sparse cones which lie amongst them ; and the rods being more 

 sensitive than cones to light of low intensity, the disc looks 

 brighter. If, while the observer is still gazing fixedly at a spot 

 to the side of the disc, the red paper be waved rapidly, but 

 gently, to right and left, a brightish grey cover seems at each 

 movement to slip off the dark green disc, and to regain its 

 position a moment later, with a jump. The grey rod-sensation, 

 developing more slowly than the green cone-sensation, is, as 



