960 VISUAL PERCEPTIONS. [BOOK in. 



sensations in a way very different from that in which objects 

 directly in the line of vision are producing sensations; it is 

 only by special analysis that we become acquainted with the 

 properties of the peripheral retina. In actual vision the activi- 

 ties of the central retina by virtue of psychical processes dom- 

 inate those of the periphery. Conversely though, as we have 

 said, when we wish to see anything very distinctly we habitually 

 make use of the central retina ; yet nevertheless in ordinary 

 vision, at the same time that we are thus making use of the 

 central retina we are also receiving impressions from the whole 

 of the rest of the retina within the field of vision, and these 

 more or less peripheral impressions influence to a certain extent 

 the psychical effect of the central sensations. Our perception 

 of an object, such as a flower, is not the same when we look at 

 it as part of a landscape, making use of the whole field of vision, 

 as when we look at it through a tube or otherwise in such a 

 way as to exclude peripheral vision ; the flower in the latter case 

 seems much more brilliant, and more highly coloured. Some 

 of the effect in this case may be physiological and due to retinal 

 events, but the greater part is psychical. The influence of 

 psychical processes is probably also illustrated by the experi- 

 ence that, if on turning our back on a landscape, we bend the 

 body so as to get a view of the landscape backwards between the 

 legs, all the objects seem to have an unusually brilliant colouring. 

 A striking difference between the objective field of sight 

 and the subjective field of vision is illustrated by the fact that, 

 though, as we have seen, that part of the retina which corre- 

 sponds to the entranc.e of the optic nerve is quite insensible to 

 light, we are conscious of no corresponding blank in the field 

 of vision. When in looking at a page of print we so direct the 

 visual axis that some of the print must fall on the blind spot, 

 no gap in the print is perceived ; we have to take special meas- 

 ures ( 573) to discover the existence of the spot. We could 

 not expect to see a black patch, because what we call black is 

 the absence of the sensation of light from structures which are 

 sensitive to light; we must have visual organs to see black. 

 But there are no visual organs in the blind spot, and conse- 

 quently we are in no way at all affected by the rays of light 

 which fall on it. By psychical operations we " fill up," as it is 

 said, the vacancy caused by the blind spot, so that there is in 

 our subjective field of vision no gap corresponding to the gap 

 in the retinal image ; we treat the sensations coming from two 

 points of the retina lying on opposite margins of the blind spot 

 as if they were sensations excited in two points lying close 

 together, thus preserving the continuity of the field of vision 

 between them. Concerning the particular psychical actions 

 by which this is carried out, and concerning the special effects 

 which are produced when an object in the field of sight passes 



