254 Prof. Wheatstone on the Physiology of Vision. 



forward, which effect gives to the whole solid a quite contrary 

 apparent inclination.'" 



Professor Necker attributes this alteration of appearance, not 

 to a mental operation, but to an involuntary change in the ad- 

 justment of the eye for obtaining distinct vision. He supposed 

 that whenever the point of distinct vision on the retina is directed 

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

 the others, is naturally supposed to be nearer and foremost, 

 while the other angles seen indistinctly are supposed to be 

 further and behind, and that the reverse takes place when the 

 point of distinct vision is brought to bear on the angle X. 



That this is not the true explanation, is evident from three 

 circumstances : in the first place, the two points A and X 

 being both at the same distance from the eyes, the same altera- 

 tion of adjustment which would make one of them indistinct 

 would make the other so ; secondly, the figure will undergo 

 the same changes whether the focal distance of the eye be ad- 

 justed to a point before or beyond 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 intended to be represented, or of its converse. By fol- 

 lowing 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 before observed, these effects are far more obvious when 

 the figures are regarded with one eye only. 



No illusion of this kind can take place when an object of three 

 dimensions is seen with both eyes while the optic axes make a 

 sensible angle with each other, because the appearance of the 

 two dissimilar images, one to each eye, prevents the possibility 

 of mistake. But if we regard an object at such a distance that 

 its two projections are sensibly identical, and if this projection 

 be capable of a double interpretation, the illusion may occur. 

 Thus a placard on a pole carried in the streets, with one of its 

 sides inclined towards the observer, will, when he is distant 

 from it, frequently appear inclined in a contrary dnection. Many 

 analogous instances might be adduced, but this will suffice to 

 call others to mind ; it must however be observed, that when 

 shadows, or other means capable of determining the judgement 

 are present, these fallacies do not arise. 



§ 11. 



The same indetermination of judgement which causes a draw- 

 ing to be perceived by the mind at different times as two dif- 

 ferent figures, frequently gives rise to a false perception when 



