THE SENSES. 



547 



if in the line of direct vision for the right eye it will be out of that line 

 for the left eye, and vice versa. There is, accordingly, only a certain 

 distance, directly in front, at which an object can be distinctly seen sim- 

 ultaneously by both eyes ; namely, that at which the two lines of direct 

 vision coincide. This point is called the point of fixation for the two 

 eyes. In fixing any object, for binocular vision, the accommodation 

 in each eye is adjusted for the required distance ; and thus the entire 

 accuracy of both organs is concentrated upon a single point. 



Since it is the position of the two eyes in their orbits which deter- 

 mines the point of fixation, the observer can form a tolerably accurate 

 judgment, as to whether another person within a moderate distance be 

 looking at him, or at a different object in the same direction. For 

 greater distances the estimate fails, because the obliquity of the eyes, 

 in looking at remote objects, is so small that its variation is no longer 

 appreciable. 



Single Vision with both Eyes. It is evident from the preceding that 

 there can be only one point in the line of direct vision for both eyes at the 

 same time. When an object occupies this situation, namely, the point 

 of fixation, it is distinctly perceived by each eye in the centre of the 

 field of vision ; thus its two visual images exactly cover each other and 

 so form but one. Consequently, the object appears single, though 

 seen by both eyes. But if placed either within or beyond the point of 

 fixation, it will appear indistinct and at the same 

 time double. If the observer hold a slender rod 

 in the vertical position at a distance of one or 

 two feet before the face, and in the same range 

 with any small object, such as a door-knob, on 

 the opposite side of the room, it will be found 

 that when both eyes are directed at the rod, it 

 is seen single and distinctly, but the door-knob 

 appears double, one of its images falling on each 

 side. If the eyes be now directed at the door- 

 knob, that in turn becomes distinct and single, 

 while the figure of the rod is double, one indis- 

 tinct image appearing on each side, as before. 



These phenomena depend on the different direc- 

 tions of the two lines of vision. When the nearer 

 object (Fig. 144, : ) occupies the point of fixation, 

 the farther object ( 2 ) will also be seen, because it 

 is still included in the visual field; though it ^L 

 will be seen indistinctly, because the accommoda- \""~J 

 tion of the eye is not adjusted to its distance, and 5 

 because it is not in the direct line of sight. But 

 for the right eye (a) it will be placed to the right 

 of this line, and for the left eye (6) to the left of it. 

 Its two images do not correspond with each othe.. 

 in situation, and it accordingly appears double. 



SINGLE AND DOUBLE VISION, 

 at different distances. a. 

 Right eye. 6. Left eye. 1. 

 Object at the point of fixa- 

 tion, seen single. 2. Object 

 beyond the point of fixa- 

 tion, seen double. 



