IN SINGLE AND BINOCULAR VISION. 36] 



objects in the most vivid manner." Having already explained, in 3, the true 

 process by which solid bodies are seen in relief, I shall now endeavour to shew, 

 that, in the vivid relief produced by the union of two dissimilar plane pictures, this 

 union is merely a necessary accessory, and not the cause of the phenomenon in 

 question. 



When two of the images of two perfectly similar objects are united either by 

 looking at a nearer or a remote object, the compound image, thus formed, is 

 seen at the place where the two optic axes converge, and is larger and more re- 

 mote than the single image if we look at a more distant object, and smaller and 

 nearer if we look at a nearer object.* The best mode of conducting this class of 

 experiments is to suspend two equal rings by invisible fibres, or to cement them 

 upon a large plate of glass, whose surface and figure are not visible to the ob- 

 server. The object of this arrangement is to prevent the observer from having 

 any knowledge of their distance from the eye. When the rings, thus placed, are 

 doubled, interpose an aperture, so as to permit only the united rings to be seen ; 

 and it will be found that they appear at the place to which the optic axes con- 

 verge, appearing smaller and nearer, or larger and more remote, according as the 

 optic axes are converged to a point nearer or more distant than the actual rings. 

 In both these cases, the similar rings are seen in identically the same manner, 

 having the same apparent magnitude and position as if a similar real ring were 

 placed as an object at the spot to which the optic axes converge. Let us now 

 apply these facts to the vision of the apparent solid produced in consequence of 

 the union of two dissimilar plane pictures of it. For this purpose, I shall take 

 the case of the frustum of a cone, after having considered the process by which we 

 see a real frustum of a cone by both eyes the nature of the compound picture 

 which we do see and the cause of the apparent single picture of which the mind 

 takes cognizance. 



When we look at the real frustum of a cone (ABCD, placed as in fig. 16), the 

 right eye R sees a solid, whose projection is a! V CD, or abed, fig, 17 ; and the 

 left eye to a solid, whose projection is A'B'CD, or ABCD, fig. 17. The smaller 

 circle CD appears nearer to the observer than the base AB, because the eye cannot 

 see it distinctly without adjusting itself to the distance RC, LD, and converging 

 its optic axes to that distance. Each eye, acting alone, sees the cone single, and 

 the various points of its outline are seen more or less distinct, according as they 

 are more or less remote from the point to which the eye is for the instant adjusted. 

 But so rapid is the motion of the eye, and so quickly does it survey the whole of 

 the solid, that it obtains a most distinct perception of its form, its surface, and its 



* Several curious facts establishing this result have been given by Dr SMITH in his Compleat System 

 of Optics, vol. ii. 387-389 ; and Remarks, 526-527. 



VOL. XV. PART III. 5 E 



