308 Sir D. Brewster on the Knovcledge of 



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 imper- 

 fectly made paper-hangings, than in those which are more 

 carefully executed. 



In continuing our survey of the suspended image, another 

 curious phaenomenon 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 imper- 

 fection 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 flower 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 further 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 VI., where M N, O 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 C, D, are united as in the figure, it is ob- 

 vious that the flower is incomplete, a part of the central circle 

 of the corolla having been cut off' from each half. If we now, 

 by straining the eye, unite C D with B, and also with A, then 

 at the same time E will be united with the second or left-hand 

 image of A, and G with the second or right-hand image of 

 B. But since a piece has been cut out of C D, the half a « of 

 A is nearer the half D D than the other half a a is to the other 

 half CC; and, in like manner, the \\a\ibb of B is nearer the 

 half CC than the other half /3/3 is to the other half D D. 

 Hence, when the strained eyes unite a « to D D, the binocu- 

 lar centre is more remote than when a a is united to C, and 

 the same is true of the other halves ; consequently', the halves 

 DD and hb must appear, as it were, sunk in the wall, or as 

 further removed from the observer; and if the defective cut- 

 ting exists along the line R S from the floor to the ceiling, 

 the whole stripe of paper between R S and O P, from the 



