368 SIR DAVID BREWSTER ON THE LAW OF VISIBLE POSITION, &c. 



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 binocular 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 illu- 

 sion 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 interesting phenomena, 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 represent in- 

 compatible solids ; to the union of dissimilar pictures, 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 union of pic- 

 tures, one of which is more or less turned round in its own plane ; to the pheno- 

 mena exhibited by uniting the images of two similar real solids, the one elevated 

 and the other depressed ; to the union of dissimilar plane figures which should 

 at the same time give a solid in relief, and in the converse of relief ;f and to the 

 union of portions of dissimilar figures, those which are wanting in the one figure 

 existing in the other. Among the singular effects produced under these various 

 conditions, nothing is more remarkable than the tendency or desire, as it were, 

 of the eyes, to unite and fix the two pictures hovering before them, to convert 

 them into some figure of three dimensions (sometimes in relief, sometimes in the 

 converse, and sometimes in both at the same time) ; and the suddenness with 

 which the two images start into union, give birth to a solid figure on which the 

 optic axes are converged, and release the eyes from that unnatural condition in 

 which they had previously been placed. 



ST LEONAKD'S COLLEGE, ST ANDREWS, 

 January 1843. 



* 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. 



f In order to produce simultaneously this double effect, the lines of the pyramid, for example, 

 which are to give the converse of relief, should be fainter than the other lines, or in different and feebler 

 colours. 



