522 VISUAL SENSATIONS. [BOOK in. 



Distinction and Fusion of Sensations. When light falls on a 

 large portion of the retina the total sensation produced is greater 

 in amount than when a small portion only of the retina is affected ; 

 a large piece of white paper produces a greater total effect on our 

 consciousness than a small one, though, if the surfaces be uniformly 

 and equally illuminated, the intensity of the sensation is in each 

 case the same ; the small piece of paper appears as bright or as 

 ' white ' as the large one. If the images of two luminous objects 

 fall on the retina at sufficient distances apart, the consequent 

 sensations are distinct, and the intensity of each sensation will 

 depend solely upon the luminosity of the corresponding object. If 

 however the two objects are made to approach each other, a point 

 will be reached at which the two sensations are fused into one. 

 When this occurs the intensity of the total sensation produced will 

 be greater than that of either of the sensations caused by the 

 single objects. A number of luminous points scattered over a wide 

 surface would appear each to have a certain brightness ; each would 

 give rise to a sensation of a certain intensity. If they were all 

 gathered into one spot, that spot would appear far brighter than 

 any of the previous points ; the intensity of the sensation would be 

 greater. We may therefore suppose the retina to be divided into 

 areas corresponding to sensational units. If the images from two 

 luminous objects fall on separate visual areas, if we may so call 

 them, two distinct sensations will be produced ; if, on the contrary, 

 they both fall on the same visual area, one sensation only will be 

 produced. Where the sensations are separate, the intensity of the 

 one (with exceptions hereafter to be mentioned) is not affected by 

 the presence of the other; but where they become fused the 

 intensity of the united sensations is greater than either of, though 

 not equal to the sum of, the single sensations. The existence of 

 these sensational units is the basis of distinct vision. When we 

 speak of the smallest size visible or distinguishable, we are referring 

 to the dimensions of the retinal areas corresponding to these sen- 

 sational units. The retinal area must be carefully distinguished 

 from the sensational unit, for the sensation is, as we have seen, a 

 process whose arena stretches from the retina to certain parts of 

 the brain, and the circumscription of the sensational unit, though 

 it must begin as a retinal area, must also be continued as a 

 cerebral area in the brain, the latter corresponding to, and being 

 as it were the projection of, the former. With most people two 

 stars appear as a single star when the distance between them sub- 

 tends an angle of less than 60 seconds ; and the best eyes generally 

 fail to distinguish two parallel white streaks when the distance 

 between the two, measured from the middle of each, subtends an 

 angle of less than 73 seconds. Some however can distinguish 

 objects 50 seconds distant from each other. An angle of 73 seconds 

 in an object corresponds in the diagrammatic eye (see p. 492) to the 



