CHAP, in.] SIGHT. 887 



as we have just seen, to the region of the retina affected, and 

 will possess features determined by the relative positions of, that 

 is, by the figure formed by the luminous points ; it will be the 

 sensation of a surface of a certain form, round, square or the like ; 

 thus the retinal area stimulated supplies data, which are used, 

 in a manner which we shall study later on, for judging the size 

 and form as well as the position of visible objects. 



When the images of two luminous points are at a certain 

 distance apart on the retina, the two sensations may have no 

 appreciable effect whatever on each other ; but when they are 

 within a certain distance from each other, the sensations do 

 affect each other, in a manner which we shall study later on. 

 Meanwhile we will merely say that when two images approach 

 so closely that the two sensations become fused into one, such 

 a mutual influence is exerted that the intensity of the total sen- 

 sation produced is greater than that of either of the sensations 

 caused by a single image, though less than the sum of the two. 

 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 brighter than 

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

 be greater. 



556. The region of distinct vision. The distance at which 

 two images must be apart from each other in order that the two 

 sensations may be separate is not the same for the whole area 

 of the retina. If two luminous points lie near the optic axis, so 

 that their images fall on the fovea centralis or on the yellow 

 spot, they will be seen as two distinct points, even when their 

 images lie very close indeed to each other. If the luminous 

 points be moved aside, so that the images fall on the retina out- 

 side the yellow spot, the two luminous points, though at the 

 same distance apart from each other, will give rise to one sen- 

 sation only, and be seen as one point ; they may be moved even 

 farther apart from each other and still give rise to one sensa- 

 tion ; and if the two points be placed so much 011 one side that 

 their respective images fall on the extreme peripheral parts of 

 the retina near the ora serrata, the two images may be separated 

 from each other a very considerable distance and yet give rise 

 to one sensation only. We may vary the experiment by making 

 use of a negative sensation, and take two black dots on a white 

 surface only just so far apart that they can be seen distinctly as 

 two when placed near the axis of vision so that their images fall 

 on or near the fovea, and then, keeping the axis fixed, move the 

 two points outwards, so that their images travel outwards from 

 the fovea ; it will be found that the two soon appear as one. 

 The two sensations become fused, as they would do if brought 

 nearer to each other in the centre of the field. The farther 



