BINOCULAR VISION 591 



7. By the intersection of objects with, the horizontal plane. Thus the 

 positions of trees in a field may be inferred with some accuracy, if the positions 

 of the roots of the trees in relationship with the boundaries of the field be 

 observed. 



8. By parallax, that is the rate of movement of objects in relationship 

 with one another. Thus if a middle plane be looked at, it will be noticed 

 that objects in a plane behind appear to move in the same direction as the 

 observer, while those in a plane in front appear to move the opposite way. 

 Even when we are standing still, we are all the time making involuntary 

 movements which cause the development of parallax. This process is prob- 

 ably one of the most important in producing the monocular effect of 

 depth. 



9. By the effort of accommodation required to sharply focus an object, 

 hi man the accommodation is found by experiment to give little or no percep- 

 tion of depth, possibly because the function is involuntary. It is thought 

 that in birds, in which the ciliary muscles are striated and are under voluntary 

 control, the accommodation may give valuable information of distance. 



All the above factors operate together to produce an appreciation of 

 distance which as a result of experience reaches a very high order, and with 

 the exception of the last two, are used by the artist to produce the effect of 

 vsolidity and realness. Any good picture shows us that the result can be very 

 convincing. 



STEREOSCOPIC VISION is the binocular perception of depth. It 

 consists of all the factors which operate in the case of each eye separately, 

 and in addition uses : 



1 . The convergence of the eye axes which is necessary in order to cause 

 images of near objects to form on the fovea simultaneously. 



2. The dissimilarity between the images which are formed on the two 

 retinae. 



That convergence has very little effect on the perception of distance 

 can be proved by placing weak prisms, either base in or base out, in front of 

 the eyes and in this way changing the convergence of the eye axes without 

 changing any other condition. It is found that the apparent positions of 

 objects are unaffected. 



That there is dissimilarity between the images formed on the retina can 

 be easily proved by experiment. Thus if the gaze be directed towards a 

 distant point, and the finger be held a short distance from the nose, the finger 

 appears to be to the right of the distant point with the right eye and to the 

 left with the left. If two photographs be taken of the same scene, but with 

 the camera, for the second photograph, three inches to one side of its position 

 for the first, it is found that, when the two negatives are placed so that 

 objects on the horizon correspond, there is a lateral difference of position 

 in the case of all other objects situated nearer to the camera. Measurement 

 shows that the nearer the object the greater the difference in position. Since 

 this is the case it is clear that only images in one plane can be formed on 

 corresponding retinal points ; images in all other planes must fall points 



