514 Prof. "Wheatstone on the Physiology of Vision. 



which is equal to the distance between them and the pictures ; 

 the perceived magnitude of the binocular image will increase with 

 the nearness of the pictures, and depends almost entirely on the 

 disassociation of the retinal magnitude from its usually accom- 

 panying inclination of the optic axes, the actual magnifying 

 power of the lenses having a very small influence. 



The sole use of the lenses is to render the rays of light parallel, 

 which it is necessary they should be for distinct vision when the 

 optic axes are parallel. When the reflecting stereoscope is em- 

 ployed, this means of magnifying the effect is not of much utility, 

 as pictures of any size may be adapted to that instrument. But 

 in the case of the refracting stereoscope it may be advantageously 

 made use of. By combining lenses with the refracting stereo- 

 scope, described in § 18, Daguerreotypes somewhat wider than the 

 width between the eyes may be employed. Sir David Brewster 

 has used, to effect the same purpose, semi-lenses with their 

 edges directed towards each other, which serve at the same time 

 to render the rays less convergent and slightly to displace the 

 pictures towards each other. Two corresponding Daguerreo- 

 types, each not exceeding in breadth the width between the eyes, 

 being placed close to each other, and viewed with lenses of short 

 focal distance, will, even without the aid of the prisms, give an 

 apparently highly magnified binocular image in bold relief. 



There is a peculiarity in such images worthy of remark; 

 although the optic axes are parallel, or nearly so, the image does 

 not appear to be referred to the distance we should, from this 

 circumstance, suppose it to be, but it is perceived to be much 

 neareiv, and indeed more so, as the pictures are nearer the eyes, 

 though the inclination of the optic axes remains the same, and 

 should therefore suggest the same distance ; it seems as if the 

 dissimilarity of the projections, corresponding as they do to a 

 nearer distance than that which would be suggested by the former 

 circumstance alone, alters in some degree the perception of 

 distance. 



I recommend, as a convenient arrangement of a refracting 

 stereoscope for viewing Daguerreotypes of small dimensions, the 

 instrument represented, PI. X II. fig.4, shortened in its length from 

 8 inches to 5, and lenses of 5 inches focal distance placed before 

 and close to the prisms. 



§22. 



I now proceed to another subject — to the consideration of 

 those phenomena which I have termed Conversions of Relief. 



In § 5 of my first memoir I noticed the remarkable circum- 

 stance, that when the drawing intended to be seen by the right 

 eye is presented to the left eye in the stereoscope, and vice versa, 



