664 SIR DAVID BREWSTER ON THE KNOWLEDGE OF 



other pair. When the observer, therefore, places himself in front of that side 

 of a papered room in which there are neither doors nor windows, and conceals 

 from his eye the floor, the roof, and the right and left hand sides of the room, 

 the whole of his retina will be covered with the images of the united plane 

 figures, and there will be no interposing objects to prevent him from judging of 

 the distance of the picture that may be presented to him. 



Let the observer, therefore, now place himself three feet in front of the 

 papered wall, and unite two of the figures, suppose two flowers, at the distance 

 of twelve inches. The whole wall will now be presented to his view, consisting 

 of flowers as before, but each flower will be composed of two flowers super- 

 imposed at the binocular centre, or the point of convergence of the optical 

 axes. If we call D the distance of the eyes from the wall or three feet, C the 

 distance between the eyes or two-and-half inches, and d the distance between 

 the similar parts of the two flowers, we shall have x the distance of the bin- 

 ocular centre from the wall, x = - = 30 inches nearly, and D - x = 6 inches, 



the distance of the binocular centre from the middle point between the two eyes. 

 Hence the whole papered wall, with all its flowers, in place of being seen, 

 as in ordinary vision, at the distance of three feet, is now suspended in the air, at 

 the distance of six inches from the observer. In maintaining this view of the wall, 

 the eye will, at first, experience a disagreeable sensation ; but after a few ex- 

 periments the sensation Avill disappear, and the observer will contemplate the 

 new picture with the same satisfaction and absence of all strain as if he were 

 looking directly at the wall itself: for there is a natural tendency in the eyes 

 to unite two similar pictures, and to keep them united, provided they are not 

 too distant. 



When this picture is at first seized by the observer, he does not, for a 

 while, decide upon its distance from himself. It sometimes appears to advance 

 from the wall to its true position in the binocular centre, and, when it has 

 taken its place, it has a very extraordinary character : the surface seems 

 slightly convex towards the eye ; it has a sort of silvery transparent aspect, 

 and looks more beautiful than the real paper; it moves, with the slightest 

 motion of the head, either laterally or to or from the wall. If the observer, who 

 is now three feet from the Avail, retires from it, the suspended wall of flowers 

 will follow him, moving farther and farther from the real wall, and also, but 

 very slightly, farther and farther from the observer : that is, the distance of the 

 observer from the real wall increases faster than the distance of the suspended 

 wall from it, according to the law expressed by the preceding formula. The bin- 

 ocular centre, therefore, recedes from the eye as the observer retires, and the strain 

 consequently diminishes. 

 in order to observe these phenomena in the most perfect manner, the paper 



