16 Mr. C. J. Monro on a case of Stereoscopic Illusion. 



directions in one of which we see it with each eye. Mr. Abbott* 

 rejects this as an explanation (pp. 106, 107), and denies it as a 

 fact (p. 107), but is not prepared to give a construction of his 

 own for the apparent place of any point whose images on the 

 two retinas are not similarly situated with reference to the yel- 

 low spots. The apparent places of all points having this pro- 

 perty he determines, as he does the apparent directions of all 

 others, by constructions which only relate to the case of an ob- 

 server looking straight before him, but, at least for the former 

 class of points, are the same as Sir David's as far as they go 

 (pp. 108, 109). Indeed Mr. Abbott would probably admit that 

 Sir David's construction was approximately accurate as a geome- 

 trical statement of the facts of ordinary vision. That, on the 

 contrary, it does not in general meet the case of that artificial 

 vision which formed the subject of Sir David's book, Mr. Abbott 

 has perhaps partly provedf ; and the demonstration is completed 

 by the fact that when stereoscopic pictures further apart than 

 the eyes are combined by divergence, the object is given in 

 relief as perfect as that of nature, and does not, in my experi- 

 ence, appear behind the observer. But even in artificial vision, 

 as far as my own eyes are concerned, the theory seems to apply 

 correctly to all cases which, according to that theory, would sug- 

 gest an idea easily adopted by the imagination. Strange to say, 

 in the simplest of these cases, whereas my experience agrees with 

 Sir David's theory, Sir David's experience seems to contradict it. 

 In describing the effect of combining an image of each section 

 of a repeated pattern with the conjugate image of another section 

 a certain distance off, he says (Stereoscope, p. 91) "the surface 

 .... seems slightly curved." To Mr. Abbott, who seems to 

 speak, like Sir David, of an apparent surface formed on this side 

 of the real one, it looks "somewhat convex" (Sight and Touch, 

 p. 122). Now if the eyes are taken to be represented by their 

 optical centres, the theory would evidently give a flat surface, 



* Sight and Touch, an attempt to disprove the received or Berkleian 

 Theory of Vision. By T. K. Abbott, Fellow of Trinity College, Dublin 

 (1864). 



f He gives a pair of dots in the middle of the page (107), and says that 

 on combining them stereoscopically we do not see the image below or 

 above the paper, but upon it. Now, in the first place, I do see the image 

 below or above the paper according as I combine the pair directly or con- 

 versely. Secondly, the proof meets only half of the proposition. It shows 

 that Sir David's conditions are insufficient under very trying circumstances : 

 it does not show that they are unnecessary. I venture to use the phrases 

 direct and converse stereoscopic combination, for which I find no equiva- 

 lents in either of the works cited. The combination is direct when the 

 right eye sees the right picture, and the left the left ; converse when the 

 right eye sees the left picture, and the left the right. 



