IN SINGLE AND BINOCULAR VISION. 353 



ciple is not correct, he gives loose reins to his scepticism, and maintains the ex- 

 traordinary 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 AE, BE, Fig. 2, towards 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 certain that this distance perceived, whether apparent or presumed, is greatly 

 below the real distance. If, then, we see the star in each of the optical axes AE, 

 BE, we should see it in each of these axes in the points e, e, which are incompa- 

 rably nearer A and B than E. Thus we should see two stars e, e, and their appa- 

 rent distance e e would be nearly equal to AB. 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 A e, Be, 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 ex- 

 actly in the optical axis, even when we look at them directly. 



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

 optics, that objects are seen in the direction of the ray which they send to the eye.' 1 ' 1 * 



It is almost impossible to believe that D'ALEMBEKT is serious in maintaining 

 these doctrines. The major proposition of his syllogism is absolutely incorrect. It 

 is not true that we see the star E nearer than it is. The eye does not see distances 

 directly : the mind only estimates them, and, according to its means of judging, it 

 forms a right or a wrong opinion. The second proposition is equally incorrect. We 

 do not see the star along the lines A , B e . We see it along the lines AE, BE, at 

 the very place where it is, and whether we consider it nearer or more remote than 

 it is, whether we think that it touches our eye, or exists at the remotest verge 

 of space, the position of the optical axis of each eye remains as before, and our 

 vision of the star is not affected by the truth or falsehood of our judgment. 



2. On the Law of Visible Direction in Binocular Vision* 



In admitting the correctness of the law of visible direction in monocular Vision, 

 which I have endeavoured to establish in the preceding section, Professor WHEAT- 

 STONE justly remarks, " that the result of any attempt to explain the single appear- 

 ance of objects to both eyes, or, in other words, the law of visible direction for 

 binocular vision, ought to contain nothing inconsistent with the law of visible di- 



* Opuscules Mathematiques, Tokn. I. Mctn. ix. iv. p. 273-4. 

 VOL. XV. PART III. 5 C 



