454 Sir D. Brewster on the Law of Position in Vision. 



clearly refers to that indistinctness of vision which arises from 

 distance on the retina from the foramen centrale, or point of 

 distinct vision. When the eyes are converged upon A, X is 

 seen indistinctly, and vice versa; and that this is Mr. Necker's 

 meaning is obvious from the following conclusion of his letter : 

 " What I have said of the solid angles is equally true of the 

 edges, — those edges upon which the axis of the eye, or the 

 central hole of the retina, are directed, will always appear for- 

 ward ; so that now it appears to me Certain that this little, at 

 first so puzzling phenomenon, depends upon the law of di- 

 stinct vision." That this is the true cause of the phamomenon 

 I have no hesitation in affirming. By hiding A with the finger, 

 or making it indistinct with apiece of dimmed glass, or throw- 

 ing a slight shadow over it, X appears forward, and continues 

 so when these obscurations are removed ; and the same effect 

 is produced by hiding X, A becoming then nearest to the eye. 

 This experiment may be still more satisfactorily made by 

 holding above the rhomboid a piece of ground glass (the 

 ground side being furthest from the eye), and bringing one 

 edge of it gradually down till it touches the point A, the other 

 edge being kept at a distance from the paper. In this way 

 A X, and all the lines diverging from A, become dimmer as 

 they recede from A, and consequently A becomes the most 

 forward point. A deep plano-convex lens, with its convex 

 side ground, will answer the purpose still better, the apex of 

 the lens being laid upon A or X ; or the effect may be still 

 further improved by making the roughness increase either 

 from the apex of a convex surface, or any fixed point of a 

 plane one. 



Following out his general opinion of the superiority of bi- 

 nocular vision, Mr. Wheatstone remarks, that the illusion 

 which we have been examining is most obvious with one eye. 

 It is not so with my eyes ; and I conceive it should not be so, 

 as the convergency of the optic axes can have no efficacy in 

 preventing illusion when the figure occupies a plane surface. 



In the course of the investigation which I have now brought 

 to a close, I have had occasion to observe many very interest- 

 ing phaenomena, which it would be out of place to describe at 

 present. They relate partly to the effects produced by uniting 

 unequal and dissimilar pictures which have a tendency to re- 

 present incompatible solids; — to the union of dissimilar pic- 

 tures, when the parts of the solid which they tend to produce 

 lie wholly or principally in a plane perpendicular to the line 

 joining the eyes and to the plane of the optic axes*; — to the 



* Such as the magnified teeth of a saw, as in fig. 14, or a thin section 

 of a hexagonal prism whose axis is parallel to a line joining the eyes. 



