SENSE OF SIGHT. 



639 



Fig. 205. 



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 respective orbits which 

 determines the point of fixation, the observer can form a tolerably accu- 

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

 be looking at him, or at some other object farther removed in the same 

 direction. For more considerable distances the estimate fails, because 

 the obliquity of the two eyes, which varies perceptibly within moderate 

 distances, diminishes so much in looking afc remote objects, that the 

 slight differences which exist are no longer appreciable by the observer. 



Single and Distinct Vision with both Eyes. From the preceding 

 facts it is evident that only one point can be found 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 both eyes in the centre of the 

 field of vision ; thus its two visual images 

 exactly cover each other in their apparent 

 position and so form but one. Consequently, 

 the object appears single, though seen simul- 

 taneously by both eyes (Fig. 205). But if 

 placed either within or bej'ond the point of 

 fixation, an object appears 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 upon each 

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

 knob, that in turn becomes distinct and single, 

 while the rod appears double, one indistinct 

 image being placed on each side, as before. 



These phenomena depend upon the different directions of the two 

 lines of direct vision. When the eyes are so directed that the nearer 

 object (Fig. 205, i) occupies the point of fixation, the farther object (a) 

 will also be seen, because it is still included in the visual field ; although 

 it will be seen indistinctly, because the accommodation of the eye is no 

 longer adjusted to its distance, and because it is not in the line of direct 

 vision. But for the right eye (a) it will be placed to the right of this 

 line, and for the left eye (b) to the left of it. Its two images do not cor- 

 respond with each other in situation, and the object accordingly appears 

 double. 



If the eyes, on the other hand, be directed at the more distant object, 



b 



SlTTGLE ATTD DOTTBLE Vl- 



SION, at different distances. a. 

 Right eye. b. Left eye. 1. Ob- 

 ject at the point of fixation, seen 

 single. 2. Object beyond the 

 point of fixation, seen double. 



