548 THE HUMAN BODY. 



gray (slightly luminous white) surface, which feebly excites 

 red, green, and violet sensations, looks deficient in red (and so 

 bluish green) near a deeper red surface. There are, however, 

 certain phenomena of simultaneous contrast which cannot be 

 satisfactorily so explained, and these have led to other theories 

 of color vision, the most important of which is that described 

 in the next paragraph. 



Hering's Theory of Vision. Contrasts can be seen with 

 the eyes closed and covered. If we look a short time at a 

 bright object and then rapidly exclude light from the eye, we 

 see for a moment a positive after-image of the object, e.g. f 

 a window with its frame and panes after a glance at it and 

 then closing the eyes. In these positive after-images the 

 bright and dark parts of the object which was looked at retain 

 their original relationship; they depend on the persistence of 

 retinal excitement after the cessation of the stimulus and 

 usually soon disappear. If an object be looked at steadily for 

 some time, say twenty seconds, and the eyes be then closed, a 

 negative after-image is seen. In this the lights and shades of 

 the object looked at are reversed. Frequently a positive 

 after-image becomes negative before disappearing. The 

 negative images are explained commonly by fatigue; when the 

 eye is closed some light still enters through the lids and ex- 

 cites less those parts of the retina previously exhausted by 

 prolonged looking at the brighter parts of the field of vision; 

 or, when all light is rigorously excluded, the self stimula- 

 tion of the visual apparatus itself, causing the idio-retinal 

 light, affects less the exhausted portions, and so a negative 

 image is produced. If we fix steadily for thirty seconds a 

 point between two white squares about 4 mm. ( inch) 

 apart on a large black sheet, and then close and cover our 

 eyes, we get a negative after-image in which are seen two 

 dark squares on a brighter surface; this surface is brighter 

 close around the negative after-image of each square, and 

 brightest of all between them. This luminous boundary is 

 called the corona, and is explained usually as an effect of 

 simultaneous contrast; the dark after-image of the square it 

 is said makes us mentally err in judgment and think the 

 clear surface close to it brighter than elsewhere; and it is 

 brightest between the two dark squares, just as a middle-sized 

 man between two tall ones looks shorter than if alongside one 

 only. If, however, the after-image be watched it will often 



