632 A TEXTBOOK OF PHYSIOLOGY 



Normally, when we look at a distant object, the eyes are parallel, 

 so that the image falls upon the same point in both eyeballs the 

 yellow spot. When looking at a close object, owing to the position 

 of the yellow spot, the eyeballs converge slightly, so that the image 

 still falls on this point of distinct vision in both eyeballs. Thus, a 

 sensation of oneness is obtained, due to the fact that the same point 

 is regarded by the yellow spots of both eyes at the same time, and the 

 attention is directed to the object looked at, and not to other objects 

 also in the field of vision. That this is the case may be proved by 

 holding vertically at different distances from, and in the middle line 

 of, the body two small rods or pencils. When we concentrate 

 attention on one, the other becomes blurred or double. This is owing 

 to the fact that, when the eyes are directed so that the rays from one 

 fall on the yellow spots of the retinae, the rays from the other fall on 

 different parts of the retinae which do not habitually work together 

 (cf. Fig. 346). Thus, when the near object appears as one, the 

 rays from the farther one fall on the inner side of each yellow 

 spot and the result is two images are seen. These appear the same 

 distance away as the near object, and when one eye is closed, the image 

 on the same side disappears, and is called homonomotfs. When the 

 distant object is seen singly, the rays from the nearer object 

 fall on the outer side of each yellow spot. The result is that 

 two objects are seen, but in this case, when one eye is closed, 

 the image of the opposite side disappears, and is known as heter- 

 onomous. 



It requires special attention to see such double images, and 

 as a rule we are not conscious of them, since attention is paid 

 only to the object looked at, and we do not at the same time try to 

 view carefully objects at two markedly different distances from the 

 eyes. If such images are formed, they are cerebrally suppressed, 

 and not seen. This is particularly the case in persons suffering 

 from squint. Generally speaking, right-handed people appreciate 

 the right image and suppress the left. The tendency to do this is 

 illustrated by asking a person having both ej-es open to point carefully 

 in the middle line of the body at a distant object, either hand being 

 used. On closing first one eye and then the other, it is found that, 

 viewed with one eye the finger is still pointing correctly, while with 

 the other eye the image of the finger has moved to one side, and the 

 finger is not pointing at the object. 



Each person has a certain visual field. This depends to a certain 

 extent upon the setting of the eyes, whether deep or protruding, 

 and also for each eye upon the shape of the brow, bridge of nose, and 

 cheek-bones. A certain part of the field of vision is common to both 

 eyes. On closing the eyes alternately, it is seen that part of the 

 visual field is common to both ej'es, but that the brows and nose cut; 

 off part of the field visible to the other eye. There is therefore 

 marked advantage in a binocular visual field. From every point in 

 this field the rays of light in a fixed position of the eyes fall upon 

 corresponding points of the two retinae. Each point in the field lias 



