in Single and Binocular Vision. 36 1 



the visible point retaining its position. Hence it follows, that 

 on the part of the retina in the axis of vision, all rays, however 

 obliquely incident, give the same visible direction perpendi- 

 cular to the surface of the membrane. That the same pro- 

 perty is possessed by every other part of the retina cannot be 

 doubted, and may be proved by direct experiment. 



Although D' Alembert states it as unquestionable, that when 

 the visual ray is in the axis of vision, or the optic axis, and 

 passes to the retina without refraction, the point which emits 

 it will be seen in the direction of a line passing from its image 

 to the visible point; yet, after he has found that his mecha- 

 nical principle is not correct, he gives loose reins to his scep- 

 ticism, and maintains the extraordinary paradox, that objects 

 even which are placed in the optical axis are not always seen 

 in this axis. The following is the argument he employs, which 

 I shall give in his own words. 



" If we direct the two optic axes A E, B E, fig. 2, towards 



Fig. 2. 



a star E, it is certain that this star appears much nearer to us 

 than it really is : it is true that we estimate its distance only 

 in a very imperfect and vague manner ; but it is not less cer- 

 tain that this distance perceived, whether apparent or pre- 

 sumed, is greatly below the real distance. If, then, we see the 

 star in each of the optical axes A E, B E, we should see it in 

 each of these axes in the points e, e, which are incomparably 

 nearer A and B than E. Thus we should see two stars e, e, 

 and their apparent distance e e would be nearly equal to A B. 

 Observation, however, proves that we see only one star, and 

 this star is seen nearly at the middle point e of the line e e in 

 the direction of lines As, B e, different from the optic axes. 

 It is true that these lines, though really different from the optic 

 axes, deviate from them but very little, but still they do differ 

 from them ; and this experiment is sufficient to prove that 

 objects which are at a considerable distance from the eye are not 

 seen exactly in the optical axis, even when we look at them di- 

 rectly. 



" Whence, in general, nothing is less certain than this 



