THE SENSES 123 



(2) BINOCULAR VISION. 



By the fact that there are two eyes instead of only one, 

 the following advantages are attained : 



1. The Field of Vision is increased, but is not doubled. 

 This is shown in Fig. 60, where the two eyes are directed to 

 a spot A, and where the field of vision of the right eye is 

 indicated by continuous lines, that of the left eye by dotted 

 lines. The two fields greatly overlap, and the central part 

 is common to the two eyes. (Experiment.) In animals 



\ 



; L.FV 



FIG. 60. The Field of Vision in Binocular Vision. The two eyes are directed to a 

 point, A. The field of vision of the right eye subtends the angle formed by 

 the continuous lines, and that of the left that subtended by the dotted lines. 

 The overlap of the fields is shown on the surface looked at and in the figure 

 below, R.F.V. andL.F.V. 



where the eyes are placed laterally, the two fields are 

 independent. 



2. A mechanism is afforded for the determination of the 

 distance of near objects, because as an object is approached, 

 the two eyes have to be turned inwards by the internal recti 

 muscles, and by the degree of contraction of these, an 

 estimation of the distance is made. The importance of 

 this may be demonstrated by fixing a stick vertically, 

 rapidly walking up to it with one eye shut, and endeavour- 

 ing to touch it with the finger. (Experiment.) 



3. A means of determining the solidity of an object 

 is afforded, because if the object is near, a slightly dif- 

 ferent picture is given on each retina, and experience has 



