446 Sir David Brewster on the Law of Visible Position 



both these cases, the similar rings are seen in identically the 

 same manner, having the same apparent magnitude and posi- 

 tion 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 conse- 

 quence 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 (A B C D, 

 placed as in fig. 16), the right eye R sees a solid, whose pro- 

 jection is a! b' C D, or a b c d, fig. 1 7 ; and the left eye a solid, 

 whose projection is A'B'CD, or ABC D, fig. 17. The 

 smaller circle C D appears nearer to the observer than the 

 base A B, because the eye cannot see it distinctly without ad- 

 justing itself to the distance R C, L D, 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 percep- 

 tion of its form, its surface, and its solidity. When we view 

 the cone with both eyes, we have the same indistinctness of 

 outline when the optic axes are converged to a single point : 

 but in addition to this, we have the greater indistinctness ari- 

 sing from every point of the figure being seen double, except 

 the single point to which the axes are converged. But this 

 imperfection, too, is scarcely visible, from the rapid view which 

 the eyes take of the whole solid, converging their axes upon 

 every point of it, and thus seeing each point in succession 

 single and distinct. Hence we must draw a marked distinc- 

 tion between the vision of the solid (as an optical fact) when 

 the eyes are fixed upon one point of it, and the resultant per- 

 ception of its figure arising from the union of all the separate 

 sensations received by the two eyes. 



Let ABCD, fig. 16, be the solid frustum of a cone, having 

 its axis M N produced, bisecting at O the distance L R be- 

 tween the two eyes L, R. Draw A L, A R, B L, B R ; and 

 also CL, CR, and DL, DR. Then, if we look at this solid 

 with the left eye L only, the projection of it will be as shown 

 in fig. 17 at ABCD, and in fig. 16 at A'B'CD; AC being 

 much greater than DB, and the summit-plane CD appearing 

 on the right-hand side of the centre of the base AB. The 



