in Single and Binocular Vision. 453 



from him, and that the face ACBD should be the foremost 

 while the face X D C is behind. But in looking repeatedly 

 at the same figure, you will perceive that at times the apparent 

 position of the rhomboid is so changed that the solid angle X 

 will appear the nearest, and the solid angle A the furthest, and 

 that the face A CDB will recede behind the face XDC, which 

 will come forward ; which effect gives to the whole solid a quite 

 contrary apparent inclination." Professor Necker observed 

 this change " as well with one as with both eyes," and he con- 

 sidered it as owing " to an involuntary change in the adjust- 

 ment of the eye for obtaining distinct vision ; and that when- 

 ever the point of distinct vision on the retina was directed on 

 the angle A, for instance, this angle, seen more distinctly than 

 the others, was naturally supposed to be nearer and foremost; 

 while the other angles seen indistinctly were supposed to be 

 further away and behind. The reverse took place when the 

 point of distinct vision was brought to bear upon the angle X." 

 Upon this explanation Mr. Wheatstone makes the following 

 observations: — "That this is not the true explanation is evi- 

 dent from three circumstances : in the fast place, the two 

 points A and X being both at the same distance from the 

 eyes, the same alteration of adjustment which would make one 

 of them indistinct would make the other so ; secondly, the 

 figure will undergo the same changes whether the eye be ad- 

 justed to a point before or behind the plane in which the figure 

 is drawn; and, thirdly, the change of figure frequently occurs 

 while the eye continues to look at the same angle. The effect 

 seems entirely to depend on our mental contemplation of the 

 figure, or of its converse. By following the lines with the eye, 

 with a clear idea of the solid figure we are describing, it may 

 be fixed for any length of time ; but it requires practice to do 

 this, or to change the figure at will. As I have observed 

 before, these effects are far more obvious when the figures are 

 regarded with one eye only." 



In a case of this kind, where one eminent individual assures 

 us that he has proved his explanation to be true in three dif- 

 ferent ways, and another maintains that this explanation is 

 evidently not the true one from three different circumstances, 

 there must be a misapprehension to be removed as well as a 

 difficulty to be solved. It is impossible to read Mr. Necker's 

 paper without discovering that Mr. Wheatstone has entirely 

 mistaken his meaning, though the mistake is partly owing to 

 Mr. Necker's use of the phrase, "adjustment of the eye for 

 obtaining distinct vision." Mr. Wheatstone understands this 

 to mean the adjustment of the eye to A or X, as if they were 

 at different distances from the observer ; whereas Mr. Necker 



