778 SIGHT. 



dots, only just so far apart that they can be seen distinctly as two when 

 placed near the axis of vision, and then, keeping the axis fixed, move the 

 two points out into the circumferential parts of the field of vision, 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 further away from the centre of the field, the further apart must two 

 points be in order that they may be seen as two. In other words, vision is 

 much more distinct in the centre of the field than toward the circumference. 

 Practically the region of distinct vision may be said to be limited to the 

 macula lutea, or even to the fovea centralis ; by continual movements of the 

 eye we are constantly bringing any object which we wish to see in such a 

 position that its image falls on this region of the retina. 



The diminution of distinctness does not take place equally from the 

 centre to the circumference along all meridians. The outline described by 

 a line uniting the points where two spots cease to be seen as two when moved 

 along different radii from the centre is a very irregular figure. 



The sensations of color are much more distinct in the centre of the retina 

 than toward the circumference. If the visual axis be fixed and a piece of 

 colored paper be moved toward the outside of the field of vision, the color 

 undergoes changes and is eventually lost, red disappearing first, and blue 

 last, the object remaining visible, though with very indistinct outlines, when 

 its color can no longer be recognized. A purple color becomes blue, and a 

 rose color a bluish white. In fact, there seems to be a certain amount of 

 red-blindness in the peripheral parts of all retinas. 



Modified Perceptions. 



674. Since our perception of external objects is based on the distinct- 

 ness of the sensations which go to form the perception, it might be expected 

 that when an image of an object is formed on the retina the sensory impulses 

 would correspond to the retinal image, the sensations correspond to the sen- 

 sory impulses and the perception correspond to the sensations, and that, 

 therefore, the mental condition resulting from our looking at any object or 

 view would correspond exactly to the retinal image. We find, however, that 

 this is not the case. The sensations and probably even the simple sensory 

 impulses produced by an image react upon each other, and these reactions 

 modify our perceptions, independently of the physical conditions of the 

 retinal image. There arise certain discrepancies between the retinal image 

 and the perception, some having their source in the retina, some in the brain, 

 and others being of such a nature that it is difficult to say where the 

 irrelevancy is introduced. 



675. Irradiation. A white patch on a dark ground appears larger, 

 and a dark patch on a white ground smaller, than it really is (Fig. 177). 

 This is especially so when the object is somewhat out of focus, and may in 

 this case be partly explained by the diffusion circles which, in each case, 

 encroach from the white upon the dark. But over and beyond this, any 

 sensation coming from a given retinal area occupies a larger share of the 

 field of vision, when the rest of the retina and central visual apparatus are 

 at rest, than when they are simultaneously excited. It is as if the neighbor- 

 ing, either retinal or cerebral, structures were sympathetically thrown into 

 action at the same time. 



676. Contrast. If a white strip be placed between two black strips, 

 the edges of the white strip near the black will appear whiter, than its 

 median portion ; and if a white cross be placed on a black background, the 

 centre of the cross will appear sometimes so dim, compared with the parts 



