42 J. LeCoate on some phenomena of Binocular Viston. 



well without the screen, for in that case only the right branci 

 tan be seen by the right eye, (the left branch being covered by 

 it), and the left branch by the left eye. If the compass be 

 placed a little below the plane of vision, so that both branclies 

 may be seen by each eye, or better if it be laid on the plane 

 before described as in fig. 27, then the visual result will be 

 represented by fig. 28. 



The error in the interpretation of this very simple experi- 

 ment, is due to the difficulty which most persons, even good 

 observers, experience in separating what they know from what 

 they see. In this experiment we know that the nose is in tbe 

 middle between the eyes ; we know also that the head of tbe 



and we see that the two points of the 

 ^ ;e see a closed 



compass stretching from the nose to this point. But this i 



compass are together, therefore we 





only different from what we really see when our visual impres- 

 sions are properly analysed, but is contrary to the first principles 

 of binocular vision. The head of the compass impresses the 

 extreme temporal limits of the retinae of the two eyes, and 

 must therefore not only be seen double, but the double images 

 are separated by the whole coiamon field of view. It seems almost 

 incredible that images so widely separated should be imagined 

 single. 



Again : m Guy's Hospital reports is found an elaborate series 

 of papers on the " Physiology of Vision," by Joseph Townes, 

 extending through 8 vols., viz: 3d ser., viii and xv inclusive 

 Mr. Townes' experiments are performed with great care, many of 

 them with the apparatus, and in the manner, already described. 

 He has also in most cases truly described and truly drawn his 

 visual impressions, as far as he has distinctly perceived them. But 

 on account of the faulty mode of representation he seems to 

 have neglected one half of the visual impressions which he 

 ought to have perceived. His papers also show misconception 

 on some fundamental points in binocular vision which I cannot 

 now notice. I wish now only to compare his visual results 

 with my own, and thus to show the superiority of my mode oi 

 representation. Among his many figures I will select only 

 two, as the principles involved are precisely the same in all. 



In the accompanying figure (fig. 29) I reproduce his drawing 

 fig. 1. PI. I, only changing the lettering so as to bring it mto 

 accord with the previous figures of this paper. The visual re- 

 sult which he obtains when this drawing is viewed on the plane 



already described, the point of sight being at A, is given in fig- 

 30. In this result V is the combined visual lines vr and vh " 

 IS the median line B of the drawing, as seen by the right eye, 

 and b the same line as seen by the left eye, but the right eye 

 image of v I and the left eye image of v r are entirely overlooked. 



