156 VISUAL PEKCEPTIONS. [BOOK in. 



that objects on the far left or far right hand are producing 

 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 activities 

 of the central retina by virtue of psychical processes dominate 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 im- 

 pressions 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 experience 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 corresponds to the 

 entrance 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 measures ( 770) 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 consequently 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 ac- 

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

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



