io6o 



VISION. 



after-image of a black patch on a white ground, the black corona of the 

 light after-image is much less marked. Similar appearances occur in 

 coloured images. 



On the other hand, the after-image change in a limited part of the 

 visual field is influenced by the brightness of the surrounding parts. 

 This was described by Hering under the name of induction. When 

 black and white surfaces adjoin one another, the black may be observed 

 to become brighter and the white darker on fixation. A grey patch on 

 a ground brighter than itself will appear to become lighter when fixed ; 

 on a ground darker than itself it will appear to become darker. Martius 

 has recently called attention to this phenomenon, and proposed it as a 

 method of colour photometry (p. 1078). 



Contrast The facts just given are closely connected with the 

 subject of contrast. These appearances may be regarded as examples 

 of reciprocal action of retinal areas on one another, and this reci- 

 procal action is the basis of contrast. Contrast may be simultaneous 

 or successive. Two adjoining or neighbouring parts of the visual field 

 are influenced in their brightness and colour by this reciprocal action. 

 This action is so rapid as to be almost instantaneous, and is then known 

 as simultaneous contrast. During longer observation, the difference 

 between the two parts of the field may become considerably greater, and 

 this successive contrast is due, partly to the change mentioned at the 

 end of the last section (Hering's successive induction), partly to move- 

 ment of the eye, and consequent development of after-image effects. 



Pure simultaneous contrast is most marked at first, and if steady 

 fixation is kept up, rapidly diminishes. It is also most marked when 

 the contrasting surfaces join one another. This maximum effect of 

 contrast at the line of junction, however, must 

 not be confused with marked borders, which 

 occur during fixation, and are due to slight 

 involuntary movements of the eyes, and con- 

 sequent stimulation by one surface of the border 

 of the retinal area previously exposed to the 

 other surface. 



Simultaneous contrast occurs both for light 

 and colour. Simultaneous light contrast is shown 

 in a striking manner by an arrangement, such as 

 in Fig. 384, in which a grey cross is placed on a 

 surface, half of white paper and half of black 

 velvet. The two parts of the cross will be seen 



to differ greatly in apparent brightness. A grey, observed against 

 a variable background, changes in apparent brightness with every 

 change of the background ; and it may be shown, by looking through 

 a small aperture, that these changes in brightness are not due to 

 variations in the size of the pupil. It is an interesting example of 

 light contrast, that a change in the illumination of a room which is not 

 directly observed, may show itself as a change in the apparent brightness 

 of a shadow, although there may have been no change in the actual 

 illumination of the shadow. The delicacy of Bumford's method of 

 photometry depends on contrast, the shadows showing slight changes 

 of brightness much more readily than the surrounding illuminated 

 surface. 



Simultaneous colour contrast shows itself most readily on a grey 



