768 SIGHT. 



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 sen- 

 sations 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 sensa- 

 tions 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 

 sensational 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 circumscrip- 

 tion 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 subtends 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. 745) to the length of 5.36 // in the retinal image, 1 and one of 50 seconds 

 to 3.65 <j.. 



660. In the human eye 50 cones may be counted along a line of 200 />. 

 in length drawn through the centre of the yellow spot ; this would give 4 PL 

 for the distance between the centres of two adjoining cones in the yellow 

 spot, the average diameter of a cone at its widest part being 3 //, and there 

 being slight intervals between neighboring cones. Hence, if we take the 

 centre of a cone as the centre of an anatomical retinal area, these anatomical 

 areas correspond very fairly to the physiological visual areas as determined 

 above. That is to say, if two points of the retinal image are less than 4 /;. 

 apart, they may both lie within the area of a single cone; and it is just 

 when they are less than about 4 ;j. apart that they cease to give rise to two 

 distinct sensations. It must be remembered, however, that the fusion or 

 distinction of the sensations is ultimately determined by the brain and not 

 by the retina. Two points of the retinal image less than 4 /j. apart might 

 lie both within the area of a single cone ; but the reason why, under such 

 circumstances, they give rise to one sensation only is not because one cone- 

 fibre only is stimulated. Two points of a retinal image might lie, one on 

 the area of one cone and another on the area of an adjoining cone, and still 

 be less than 4 // apart; in such a case two cone-fibres would be stimulated, 



1 By M is meant the micromillimeter, one one-thousandth of a millimeter. 



