SENSE OF VISION ESTIMATION OF SIZE. 



769 



process, seems to be a necessary inference from the following very ingenious 

 experiments, made by Professor Wheatstone with a modification of his Mirror- 

 Stereoscope, devised for separately testing the influence of the two conditions, 

 namely, the magnitude of the retinal picture and the degree of convergence 



'of the optic axes, which are ordinarily in action together. When an ob- 

 ject is moved nearer to or farther from the eye, its perceived or estimated 

 magnitude undergoes no change. But if two pictures, placed in the mirror- 

 stereoscope, be made to move to and from the mirrors, in such a manner as 

 to vary their distances from these (and therefore from the eyes), without 

 altering the angle of convergence, their perceived magnitudes are augmented 

 and reduced, in precise proportion to the increased and diminished sizes of 

 the retinal pictures. Conversely, if the two pictures be made so to change 

 their places in regard to the mirrors (by moving in a horizontal circle, of 

 which the middle point between the mirrors is the centre), that the angle of 

 convergence is increased or diminished, as it would be if the object were 

 brought nearer to the eyes or removed farther from them, the perceived 

 magnitude of the pictures is altered in an inverse manner; being reduced 

 when the angle of convergence is increased, and increased when the inclina- 

 tion of the optic axes is lessened so as to approach parallelism. Thus it ap- 

 pears that the absence of alteration in the perceived magnitude of an object 

 as ordinarily seen at varying distances, is the result of the inverse action of 

 these two kinds of suggestion ; for the enlargement of the retinal picture 

 when acting alone, occasions an increase in the perceived magnitude, whilst 

 an increase of convergence, taking place by itself, diminishes the perceived 

 magnitude; and thus as these alterations occur simultaneously when an ob- 

 ject is approximated to the eye, its dimensions seem to undergo no change; 

 as will also be the case when, by the removal of the object to a greater dis- 

 tance, these conditions are again made to vary simultaneously, though in a 

 contrary direction. It may further be remarked, that in the first of the fore- 

 going experiments, the picture whose perceived magnitude is undergoing 

 enlargement or diminution in consequence of the alteration of its retinal 

 magnitude, seems evidently to be approaching or receding; yet if we fix our 

 attention on it when it is stationary, at any instant, it appeai-s to be at the 

 same distance at one time as at another, the effect being very much like 



