Distance given by Binocular Vision. 309 



floor to tlie ceiling, will appear sunk in the papered wall. 



But if the defect is confined to a portion only of the flowers, 



then a rectangular space of the breadth R O, and of a height 



equal to the defective portion, will appear sunk in the paper. 



If every junction has the same defect as that at R S, then the 



whole will appear to consist of equal stripes, every alternate 



one being raised and the other depressed. 



In the preceding example, there are only two flowers in a 



breadth, and their distance is lOi inches, which is also the 



breadth of the sunk stripes. But if the flowers are three or 



21 21 

 four in number, and their distance — , — inches, the sunk 



3 4 



stripes will vary according as we unite two flowers whose 



distances are in the one case 7 or 14 inches, and 5^ or 10^ 



or 16f or 21 in the other. Calling B the breadth of the 



paper, n the number of flowei's or figures in that breadth, and 



W the width of the sunk stripe, then we have W = — or — - 



11 n 



SB 

 or — according as we unite the two nearest, or the first and 

 n " ' 



second flower, the first and third, or the first and fourth. 



When W=B, the sunk stripes will cover the whole paper, 



and all the flowers will lie in the same plane. 



These results afford an accurate method of examining and 

 discovering defects in the workmanship of paper-hangers, 

 carpet-makers, painters, and other artists whose profession it 

 is to combine a series of similar patterns in order to form a 

 uniform and ornamental surface. The smallest defect in the 

 similarity and equality in the figures or lines which compose 

 a pattern, and any difference in the distance of the single 

 figures, is instantly detected ; and what is remarkable, a small 

 inequality of distance in a line perpendicular to the axis of 

 vision, or in one dimension of space, is exhibited in a magni- 

 fied form as a distance coincident with the axis of vision, and 

 in an opposite dimension of space ! 



At the commencement of this class of experiments, it is 

 difficult to realize, and very easy to dissolve, the singular 

 binocular picture which we have been describing; but after the 

 eyes have been drilled for a while to this species of exercise, 

 the pictures become very persistent. Although the air-sus- 

 pended image might be expected to disappear after closing 

 one eye, and still more after having closed and re-opened 

 both, yet I have found it in its original position in this latter 

 case, and even after rubbing my eyes and shaking my head ; 

 and I have sometimes experienced a difficulty in ascertaining, 



