306 Sir D. Brewster on the Knowledge of 



after uniting these figures by binocular vision, and concealing 

 the two outstanding single figures, I obtained results which, 

 though not entirely satisfactory, proved that there existed 

 some disturbing cause which prevented the united image from 

 placing itself in the binocular centre, or the intersection of the 

 optical axes. This disturbing cause was simply the influence 

 of other objects in the same field of view, whose distance was 

 known to the observer. 



In order to avoid all such influences, and to study the sub- 

 ject under a more general aspect, it occurred to me that these 

 objects would be gained by using a numerous series of plane 

 figures, such as those of flowers or geometrical patterns upon 

 carpets or paper-hangings. These figures being always at 

 equal distances from each other, and almost perfectly equal 

 and similar, the coalescence of any pair of them, by directing 

 the optic axes to a point between the paper-hangings and the 

 eye, is accompanied with the coalescence of every 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, sup- 

 pose 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 

 superimposed at the binocular centre, or the point of conver- 

 gence of the optical axes. If we call D the distance of the 

 eves 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 binocular centre from the wall, x=p—. = 30 inches 



nearly, and D— .r=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 thr-ee 

 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 experiments the sensadon will disappear, and the observer 

 will contemplate the new picture with the same satisfaction 



