CHAP, in.] SIGHT. 



the visual axis does not coincide with, and is different from 

 the optic axis. When with both eyes we look straightforwards 

 to the far distance, the visual axes of the two eyes are parallel ; 

 when we direct the two eyes to the same fixed point, the two 

 visual axes' converge to the fixed point, the amount of con- 

 vergence being the greater the nearer the fixed point to the 

 observer. 



The horizontal plane in which the two visual axes lie is 

 called the visual plane ; and a vertical plane at right angles to 

 this, midway between the two eyes, or more exactly bisecting 

 a line, sometimes called the " base line " or " fundamental line" 

 joining the nodal points of the two eyes, is called the median 

 plane. 



786. As we have seen, the sum of the sensations which we 

 can receive from the retina at the same time is spoken of as 

 the " visual field," or " field of vision." The term therefore has 

 properly a subjective meaning, but it is sometimes used in an 

 objective sense to denote the space or area of the external 

 world, rays of light from which are capable of exciting the retina 

 at any one time ; where we wish to distinguish between the 

 two, we may call the latter the " field of sight." The dimensions 

 of the field of sight for one eye will even in the same individual 

 vary with the width of the pupil and other dioptric arrange- 

 ments of the eye ; individual variations are also considerable ; 

 but the ordinary dimensions may be stated as subtending an 

 angle of about 145 in the horizontal and about 100 in the 

 vertical meridian, the former being distinctly greater than the 

 latter. When an external object lies outside the area sub- 

 tending these angles we say that it is outside the field of sight 

 for that position of the eye ; it may of course be brought into 

 the field of sight by moving it or by moving the eye. The 

 outline of the field is an irregular one, and stretches farther 

 towards the temporal side of the fixed point, that is, towards 

 the nasal side of the retina, than on the other side ; it is some- 

 what larger and of a different form when the eye is turned 

 towards the temporal side than when the eye is directed straight 

 forwards, cf. Fig. 152. It will be understood that the two visual 

 fields of the two eyes are unlike, cf. Fig. 153. 



When we use both eyes a large part of the visual field of 

 each eye overlaps that of the other ; that is to say, the rays of 

 light proceeding from a large part of the field of sight of each 

 eye fall upon and affect both retinas. But at the same time 

 a certain part of each visual field does not so overlap any part 

 of the other. If the right hand be held up above the right 

 shoulder and brought a little forward it soon becomes distinctly 

 visible to the right eye, it enters into the field of sight of the 

 right eye. But if the right eye be closed, the right hand kept 

 in the former position is not visible to the left eye; it is 



