DISTANCE GIVEN BY BINOCULAR VISION. 667 



describing ; but after the eyes have been drilled for a while to this species of 

 exercise, the pictures become very persistent. Although the air-suspended 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, after these operations, whether 

 it was the real or the air-suspended wall that was before me. On some occasions 

 a singular effect was produced. When the flowers on the paper are distant six 

 inches, we may either unite two six inches distant, or two twelve inches distant. 

 In the latter case, when the eyes have been accustomed to survey the suspended 

 picture, I have found that, after shutting and opening them, I neither saw the 

 picture formed by the two flowers twelve inches distant, nor the papered wall 

 itself, but a picture formed by uniting the flowers six inches distant ! The bin- 

 ocular centre had shifted its place, and instead of advancing to the wall, as is 

 generally the case, and giving us ordinary vision of it, it advanced exactly as much 

 as to unite the nearest flowers, just as on a ratchet wheel the detent slips over 

 one tooth at a time ; or, to speak more correctly, the binocular centre advanced 

 in order to relieve the eyes from their strain, and when the eyes were opened, 

 it had just reached that point which corresponded with the union of the flowers 

 six inches distant. 



In the construction of complex geometrical diagrams consisting only of fine 

 lines, and in which similar figures are repeated at equal distances, it is very 

 difficult to attain minute accuracy. The points of the compasses sink to different 

 depths in the paper, and the lines which join such points seldom pass through 

 their centres. Hence arises a general inaccuracy which the eye cannot detect ; 

 but if we examine such diagrams by strained binocular vision, their imperfections 

 will be instantly displayed. Some parts will rise higher than others above the 

 general level, and the whole will appear like several cobwebs placed at the 

 distance of a tenth or a twelfth of an inch behind each other.* 



In all the experiments made by Mr WIIEATSTONE by the stereoscope, and in 

 those described in my former paper, the dissimilar figures are viewed in a direc- 

 tion perpendicular to the plane on which they are drawn. A series of very 

 interesting results, however, are obtained by uniting the images of lines meeting 

 at an angular point, when the eye is placed at different heights above the 

 plane of the paper, and at different distances from the angular point. 



Let A C, B C be two lines meeting at C, the plane passing through them being 

 the plane of the paper, and let them be viewed by the eyes at E'", E", E', E at 

 different heights in a plane G M N perpendicular to the plane of the paper. 



* This effect is finely seen in the diagram of the Homogeneous Curve, which forms Plate IX. of Mr 

 HAY'S work " On the Harmony of Form." 



VOL. XV. PART IV. 8 S 



