Prof. Wheatstone on the Physiology of Vision. 507 



tain adjustment of the arms at which the binocular image will 

 appear of its natural size, that is, the size we judge the picture itself 

 to be when we look at it directly ; in this case the magnitude of 

 the pictures on the retime and the inclination of the optic axes 

 preserve their usual relation to each other. If now the arms be 

 moved back, so as to cause a less convergence of the axes, the 

 image will appear to increase in magnitude until the arms are 

 in a straight line and the optic axes are parallel ; and, on the 

 other hand, if the arms be moved forwards, so as to form a less 

 angle, the optic axes will converge more, and the image will 

 appear gradually smaller. In this manner, while the retinal 

 magnitude remains the same, the perceived magnitude of the 

 binocular object varies through a very considerable range. 



The instrument being again adjusted so that the image shall 

 be seen of its natural size; on sliding the pictures nearer the 

 mirrors its perceived magnitude will be augmented, and on sli- 

 ding them from the mirrors it will appear diminished in size. 

 During these variations of magnitude the inclination of the optic 

 axes remains the same. 



The perceived magnitude of an object, therefore, diminishes 

 as the inclination of the axes becomes greater, while the distance 

 remains the same ; and it increases, when the inclination of the 

 axes remains the same, while the distance diminishes. When 

 both these conditions vary inversely, as they do in ordinary vision 

 when the distance of an object changes, the perceived magnitude 

 remains the same*. 



Before I proceed further it will be proper to explain the mean- 

 ing of some of the terms I employ. I call the magnitude of the 

 object itself, the real or objective magnitude ; the magnitude of 

 the picture on the retina, the retinal magnitude ; and the mag- 

 nitude we estimate the object to be from its retinal magnitude 

 and the inclination of the optic axes conjointly, I name the per- 

 ceived magnitude. I do not use the term apparent magnitude, 

 because, according to its ordinary acceptation, it sometimes 

 means what I call retinal, and at other times what I name per- 

 ceived magnitude. 



We have seen in what manner our perception of magnitude is 

 modified by the new associations which this instrument enables 

 us to form ; let us now examine how our perception of distance 



* Several cases of the alteration of the perceived rrmgpifairie of ob- 

 jects are mentioned by Dr. K. Smith (Complete System of Optics, 1738, 

 vol. ii. p. 388, imil rem. 526 anil 632)j anil Dr. It. Darwin ( Philosophical 



Transactions, vol. lxxvi. p. 313) observed that when an ocular spectrum 

 was impressed on both eyes it appeared magnified when they were directed 

 to a wall at a considerable distance. The facts noticed by these authors 

 are satisfactorily explained by the above considerations. 



