CHAP, in.] i SIGHT. 1217 



is that we cannot see things on the right-hand side ; they do not 

 tell us anything about the retina itself; they cannot even tell us 

 whether the deficiency of vision is due to changes failing to be set 

 up in the retina or to the cerebral centres failing to be affected by 

 the retinal changes; such questions we have to decide by some 

 other means than a simple examination of our sensations, and by a 

 similar roundabout way only are we able to conclude that in such 

 a hemiopia it is the nasal side of the right retina, and the temporal 

 side of the left retina, which fail to give rise to visual sensations. 

 Our sensations, in fact, tell us of themselves nothing about the 

 optical image on the retina ; they do not tell us whether the retinal 

 image is inverted or no; the fact that the retinal image is an 

 inverted one does not in itself influence our visual sensations, and 

 hence the inversion needs no correction on our part. 



752. As we have just said, if the images of two luminous 

 objects, two luminous points, fall on the retina at a certain distance 

 apart, the consequent sensations are distinct. 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. Two stars at a 

 certain distance apart may be seen distinctly as two stars, while 

 two stars nearer each other appear to be one star; we cannot 

 analyse the latter sensation into its constituent parts. 



Similarly, when the images of a number of luminous points, of 

 equal luminosity, fall on the retina sufficiently near each other, 

 the effect is not a number of sensations of luminous points, but 

 one sensation, that of a luminous surface. This introduces a new 

 feature of visual sensations, namely, that of size. If the luminous 

 points be few, so as to involve a small area of the retina, the 

 sensation is that of a small surface ; if the luminous points, equally 

 near to each other as before, be numerous, so as to involve a large 

 area of the retina, the sensation is that of a large surface. 

 Moreover, such a sensation of a surface will be referred to some 

 position in space corresponding, as we have just seen, to the region 

 of the retina affected, and will possess features determined by the 

 relative positions and so 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 

 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 sensa- 

 tions become fused into one, such a mutual influence is exerted 

 that the intensity of the total sensation produced is greater than 

 that of either of the sensations caused by the single image, though 



