IN SINGLE AND BINOCULAR VISION. 307 



dent from three circumstances : in the first 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 adjusted 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 different ways, and another maintains 

 that this explanation is evidently not the true one from three different circum- 

 stances, 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 dis- 

 tinct 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 ; Avhereas 

 Mr NECKER clearly refers to that indistinctness of vision which arises from dis- 

 tance on the retina from the foramen centrale, or point of distinct vision. When 

 the eyes are converged upon A, X is seen indistinctly, and vice versa; and that his 

 is Mr NECKER'S meaning is obvious from the following conclusion of his letter : 

 " What I have said of the solid angles is equally true of the edges, those edges 

 upon which the axis of the eye, or the central hole of the retina, are directed, will 

 always appear forward ; so that now it appears to me certain that this little, at 

 first so puzzling, phenomenon, depends upon the law of distinct vision." That 

 this is the true cause of the phenomenon I have no hesitation in affirming. By 

 hiding A with the finger, or making it indistinct with a piece of dimmed glass, or 

 throwing a slight shadow over it, X appears forward, and continues so when these 

 obscurations are removed ; and the same effect is produced by hiding X, A be- 

 coming then nearest to the eye. This experiment may be still more satisfactorily 

 made by holding above the rhomboid a piece of ground glass (the ground side 

 being farthest from the eye), and bringing one edge of it gradually down till it 

 touches the point A, the other edge being kept at a distance from the paper. In 

 this way AX, and all the lines diverging from A, become dimmer as they recede 

 from A, and consequently A becomes the most forward point. A deep plano- 

 convex lens, with its convex side ground, will answer the purpose still better, the 

 apex of the lens being laid upon A or X ; or the effect may be still farther improved 



