526 THE HUMAN BODY. 



the eyes closed and covered. If we look a shore 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. 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 persist- 

 ence 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 excites 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 

 proper stimulation 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. (I 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 bound- 

 ary 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 along- 

 side one only. If, however, the after-image be watched it 

 will often be noticed not only that the light band between 

 the squares is intensely white, much more so than the normal 

 idio-retinal light, but, as the image fades aways, often the 

 two dark after-images of the squares disappear entirely with 

 all of the corona, except that part between them which i* 



