DISTANCE GIVEN BY BINOCULAR VISION. 



should be pasted upon a large screen, previously unseen by the observer, uncon- 

 nected with the roof or the floor, and placed in a large apartment. The decep- 

 tion 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 picture, 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 phenomena 

 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 farther out 

 than another, or the flower will appear thicker and more solid, deviating con- 

 siderably from the plane representation 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 dimensions in the picture. If the dis- 

 tance, 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 two corresponding leaves is greater than the 

 distance between other two corresponding leaves, then the two first, when united, 

 will appear nearer the eye than the other two, and hence the appearance of a 

 solid flower is partially given to the combination. These effects are better seen 

 in old and imperfectly made paper-hangings than in those which are more care- 

 fully executed. 



In continuing our survey of the suspended image, another curious phenomenon 

 presents itself : a part of one of the pieces of paper, and sometimes a whole stripe 

 from the roof to the floor, will retire behind the general plane of the image, or 

 rise above it ; thus displaying, on a large scale, an imperfection in the workmanship 

 which it would have required a very narrow inspection to discover. This defect 

 arises from the paper-hanger having cut off too much of the white margin of 

 one or more of the adjoining pieces, so that when the two halves of a flower 

 are united, part of the middle of the flower is left out ; and hence when this 

 defective flow*er is united with the one on the right hand of it, and the one 

 on the left hand united with the defective one, the united or corresponding 

 portion, being at a less distance, will appear farther from the eye than those 

 parts of the suspended image composed of complete flowers. In like manner, 

 if the two portions of the flowers are not brought together, but separated by 

 a small space, the opposite effect will be produced. This will be understood from 

 Fig. 1 (Plate 17), where M N, P represent portions of two separate pieces of paper, 

 each twenty-one inches wide. In this specimen, there are only two flowers in 

 each piece, namely one white flower, A or B, and two halves. If the two halves 



