THE SENSE OF VISION 633 



a corresponding point in each retina, so that the yellow spots cor^ 

 respond, and also the inner and outer, upper and lower, parts of each 

 retina. For this reason, the projections of the objects by the cerebral 

 apparatus upon the visual field correspond in position, and are seen 

 single. This is facilitated by the fact that the fibres from the retinae 

 have a special arrangement, and connect with definite brain areas 

 (see p. 712). 



A line joining all the points which appear single in the field of 

 vision isknown as the horopter. It assumes various forms for different 

 positions of the eyes. When the visual lines are parallel, as in the 

 case of very distant objects, the horopter is a plane at infinite distance 

 coinciding with the ground. When a near object is being looked at, 

 and the visual lines converge as, for example, in looking at 

 points A and B (Fig. 358) then the horopter takes the form of 

 a circle. 



Visual Judgments The Perception of Distance, Size, and Solidity. 



" Judgments " are performed in the brain, and ar.e in some ways 

 more correctly dealt with when considering the cerebral functions. 

 Of all judgments, the visual are perhaps the most easy of analysis. 

 If we were born blind, and suddenly acquired the power of vision, 

 we should not immediately recognize everything. In our first applica- 

 tion of the sense of sight, we make great use of touch to ascertain 

 the detailed outline of objects. By this means we arrive at a correct 

 understanding of these objects, and remember them later. This is 

 well illustrated by cases of congenital blindness in which vision has 

 been restored^ by operation after as long as twenty-eight years. In 

 one such case, aged eighteen, when the bandage was first removed 

 after the operation, the patient, in reply to the question as to whether 

 the surgeon had a beard, asked to be allowed to feel it, and only on 

 touching it replied unhesitatingly, " Yes." On being shown a chair, 

 he said he saw it, but could not tell what it was without touching it. 

 He made many wild guesses as to what an apple was, but on touching 

 it said immediately: " Just fancy, an apple !" 



Recognition of colours is soon learnt. The appreciation of space, 

 size, and distance, comes but slowly. Such patients at first walk 

 warily, with hands extended, like a blind man. One patient of twenty- 

 eight years, in taking his walks by moonlight, kept jumping every 

 now and again over objects lying in his path. These were shadows 

 which he was doing his best to avoid, and he would not believe that 

 there were no obstacles in his way until he had satisfied himself by 

 touch that the ground was in reality quite flat. 



A patient of eight years, when taken on a moonlight night to 

 the veranda illuminated by three low-hanging arc lamps, thought 

 the veranda was covered with snow, and thought the moon to be an 

 arc lamp quite near him. 



Such cases* also show the interesting fact that objects are seen 

 at once in their proper position, and not upside down. The first two 

 patients mentioned were each taught the colour of two strips of paper 

 red and green. They were then placed one above the other, and 



