THE SUBJECTIVE PHENOMENA OF VISION 



575 



read in each second, and that the eight words had an average of forty letters. 

 It is clear from this that between eight and forty different impressions must 

 be presented to consciousness in each second. The function of the after 

 image in preparing the retina for the reception of new impressions would 

 therefore appear to be a very important one. The effects of simultaneous 

 contrast are equally important to vision, because the changes produced 

 by it are such that the images falling on contiguous portions of the 

 retina are made as unlike as possible. Not only are the intensity and colour 

 of adjacent parts of the image made more definite (this process being compar- 

 able to the effects of intensification on a photograph), but the blurring at the 

 edges of contours due to imperfections in the image formed on the retina are 

 also largely eliminated (this comparing with retouching in photography). 

 BINOCULAR RIVALRY must be briefly referred to here because of 

 the similarity which it shows with the phenomena described above. 

 If for any reason the images formed on the retina have 

 dissimilar contours, rivalry ensues, first one image and then the other 

 reaching consciousness. This process usually occurs independently in differ- 

 ent parts of the field, so that the visual impression consists of a patchwork 



LEFT EYE 



RIGHT EYE 



COMBINED IN STEREOSCOPE 



FIG. 297. Diagram to show how the binocular combination of 

 two dissimilar images produces a fluctuating image con- 

 taining parts of both of them. 



of the two images. Seldom if ever are both images seen in the same 

 part of the retina at the same time. It is found that a number of 

 factors can cause one image totally to suppress the other ; these are interest, 

 novelty, and brightness. The importance of this suppression can be apprecia- 

 ted by picturing the confusion which would occur if two different images were 

 simultaneously presented to consciousness, as would happen in animals in 

 which different images are formed in the two retinae and in cases of strabismus 

 in man. The parallelism between this process and those which w^e have already 

 described can be traced by regarding for one moment one of the images as 



