126 



IRRADIATION. 



[BOOK in. 



Other differences are either clearly or possibly of physiological 

 origin ; the view may at least be argued that they arise either 

 during the retinal changes through which visual impulses are 

 developed or during the subsequent cerebral changes, spoken of 

 above, through which the visual impulses give rise to visual 

 sensations ; and it is to some of these that we with first to 

 call attention. 



780. Irradiation. A white patch on a dark ground appears 

 larger, and a dark patch on a white ground smaller, than it really 

 is. In Fig. 151, the white square on the right hand side looks 



FIG. 151. 



larger than the black square on the left hand side though both are 

 exactly of the same size. So also neighbouring white surfaces 

 tend to melt together. The effect is increased when the object is 

 somewhat out of focus, and may be then partly explained by the 

 diffusion circles which, in each case, encroach from the white upon 

 the dark. But over and beyond this, any sensation coming from 

 a given retinal area occupies a larger share of the field of vision, 

 when the rest of the retina and central visual apparatus are at 

 rest, than when they are simultaneously excited. It is as if the 

 neighbouring, either retinal or cerebral, structures were sympa- 

 thetically thrown into action at the same time. In this way a 

 certain difference is established between the retinal image and the 

 perception. 



781. Simultaneous contrast. If "a white strip be placed 

 between two black strips, the edges of the white strip, near to 

 the black, will appear whiter than its median portion ; and if a 

 white cross be placed on a black background, the parts close to 

 the black will appear sometimes so white, compared with the 

 centre of the cross, that the latter will seem dim or even shaded. 

 This effect which occurs even when the object is well in focus, 

 is spoken of as one of ' simultaneous contrast ' ; the increased 

 sensation of light which causes the apparent greater whiteness of 

 the borders of the cross is regarded as the result of the ' contrast ' 

 with the black placed immediately close to it. Still more striking 

 results are seen with coloured objects. If a book, or pencil, be 



