382 THE BODY AT WORK 



graph. When one is watching a moving point of light the 

 glowing end of a match, for example the prolongation of 

 sensation has its disadvantages ; the moving point is inter- 

 preted as a streak of light. If the illumination be very 

 brilliant, the object seen may give rise to a prolonged after- 

 image. A glance at the sun leaves in the mind for seconds, 

 or even for minutes, the image of a glowing disc. Sensations 

 due to stimulation of the yellow spot last longer than those 

 which originate in the peripheral retina. If, in a train, one is 

 being carried at a certain pace, past a fence composed of up- 

 right palings, one sees the separate slats until the eyes are 

 directed towards them, when they fuse into a continuous screen. 



The phenomena of negative or complementary images are of 

 retinal origin. The bright image of the sun, if the stimulus 

 has not been too violent, gives place to a black disc. If one 

 closes the eyes after staring at a window, a black surface crossed 

 by bright lines is seen in place of a white surface with dark 

 frames to the panes. If, after staring at a red surface, one looks 

 at the ceiling, a green patch is seen ; after yellow, blue. Every 

 colour has its complement, which may be determined in this 

 way. There is much uncertainty as to the exact terms in which 

 this phenomenon is to be accounted for, but little doubt as to 

 its being due to the peculiar mode of reaction of the retina to 

 light. Chemical substances which have been used up have 

 to be restored, and during the period in which they are coming 

 back to what may be termed a neutral condition the retina 

 delivers to the brain impulses of the opposite sign. 



Contrasts which are experienced simultaneously are more 

 difficult to understand than those which appear successively. 

 In Fig. 31 the half of the grey cross which is surrounded by 

 black appears brighter than the half which lies on white paper. 

 A grey cross on a red background looks green ; on a green 

 background, red ; on yellow, blue ; on blue, yellow. If green is 

 on red, it looks greener than if it is on white or black. These 

 simultaneous contrasts are seen best when the strength of the 

 colours is reduced by covering them with tissue-paper. It is 

 as if activity of any one part of the retina is accompanied by 

 activity of the opposite sign in the remainder. But it is unsafe, 

 in explaining our various sensations, to lay too much stress 

 on the mode of stimulation. The mind judges sensations in 



