932 



IRRADIATION. 



[Book hi. 



Our perception of a tree is in part determined by events other 

 than the actual sensations, by psychical processes arising out of 

 our previous experiences of trees, and in other ways. Some of 

 these psychical processes we shall consider a little later on. 



Other differences are either clearly or possibly of physiologi- 

 cal 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 wish first to call 

 attention. 



§ 582. Irradiation. A white patch on a dark ground ap- 

 pears larger, and a dark patch on a white ground smaller, than 

 it really is. In Fig. 155, the white square on the right hand 



Fig. 155. 



side looks 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 sympathetically thrown into 

 action at the same time. In this way a certain difference is 

 established between the retinal image and the perception. 



§ 583. 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 medium 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 



