in Single and Binocular Vision. 359 



eclipses a luminous object A, look away from the screen, and 

 the object A will appear. Keeping the head steady, place 



Fig. 1. 



another screen S'* so that, when viewed directly, it does not 

 eclipse another luminous object B, the line C S' B just grazing 

 the outer edge of B. When the screens and luminous objects, 

 therefore, are so arranged that A is invisible when the axis of 

 the eye is directed to S or to A, and B visible when the axis 

 of the eye is directed to S' or B, — then by turning the eye 

 from A to B, A will appear, and B will disappear, exhibiting 

 the curious effect of an invisible body appearing by looking 

 away from it, and of a visible body disappearing by looking 

 at it ! 



Had the eyeball M N been our hypothetical one, these 

 effects would not have been produced. All objects, near and 

 remote, would have retained their relative positions and mag- 

 nitudes during its rotation. 



Hence it follows, that we are not entitled to reject any law 

 of visible direction, because it gives a position to visible ob- 

 jects different from their real position. 



Having removed this difficulty, I proceeded to examine the 

 other data of D' Alembert. Making the eyeball and the retina 

 spherical, he assumes that the centre of the latter is equidistant 

 from the foramen centrale of the retina, and the centre of the 

 crystalline lens. This, however, is far from being the case. 

 M. Dutour, and Dr. Thomas Young, have made the centre 

 of curvature of the retina coincident with the centre of curva- 

 ture of the spherical surface of the cornea, as in our hypothe- 

 tical eye; and this centre, in place of being almost half-way 

 between the apex of the posterior surface of the lens and the 

 foramen centrale, is actually almost in contact with the latter ! 

 The dissections of Dr. Knox and Mr. Clay Wallace of New 



* The two screens S, S' may be the opposite edges of a triangular notch 

 in a card held in the hand. 



