Dista7ice given h/ Binocular Vision. 307 



and absence of all strain as it 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, pi'ovided 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 some- 

 times 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 con- 

 vex 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 wall, retires from it, the suspended wall of flowers will 

 follow him, moving further and further from the real wall, and 

 also, but ver^' slightly, further and further 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, accord- 

 ing to the law expressed by the preceding formula. The 

 binocular centre, therefore, recedes from the eye as the ob- 

 server retires, and the strain consequently diminishes. 



In order to observe these phsenomena in the most perfect 

 manner, the paper should be pasted upon a large screen, 

 previously unseen by the observer, unconnected with the roof 

 or the floor, and placed in a large apartment. The deception 

 will then be complete ; and when the picture stands suspended 

 before the observer, and within a few inches of himself, he may 

 stretch out his hand and place it on the other side of the pic- 

 ture, and even hold a candle on the other side of it, so as to 

 satisfy himself that in both cases the picture is between his 

 hand and himself. 



When we survey this picture with attention, several very 

 curious phajnomena present themselves. Some of the flowers, 

 when narrowly examined, appear somewhat like real flowers. 

 In some the stalk gradually retires from the general plane of 

 the picture; in others it rises above it: one leaf will come 

 further out than another, or the flower will appear thicker and 

 more solid, deviating considerably from the plane representa- 

 tion of it seen by each eye separately. All this arises from 

 slight and accidental irregularities in the two figures which 

 are united, thus producing an approximation to three dimen- 

 sions in the picture. If the distance, for example, of the ends 

 of two stalks in two coalescing flowers is greater than the 

 distance of corresponding points in other parts of the stalk, 

 the end of the stalk will rise from the general surface of the 

 figure, and vice versa. In like manner, if the distance between 



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