940 FIELD OF SIGHT. [BOOK in. 



from the optic axis. When with both eyes we look straight- 

 forwards 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 

 convergence 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. 



588. 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 arrangements 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 

 subtending 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. 156. It will be understood that the two 

 visual fields of the two eyes are unlike, cf. Fig. 157. 



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, we find that the right 

 hand kept in the former position is not visible to the left eye ; 

 it is outside the field of sight of that eye ; it has to be brought 



