310 Sir D. Brewster oti the Knowledge of 



after these operations, whether it was the real or the air-sus- 

 pended wall that was before me. On some occasions a sin- 

 gular 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 pic- 

 ture, 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 binocular 

 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 centime ad- 

 vanced 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 con- 

 sisting 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 diflPerent 

 depths in the paper, and the lines which join such points sel- 

 dom 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. Wheatstone by the 

 steoroscope, and in those described in my former paper, the 

 dissimilar figures are viewed in a direction perpendicular to 

 the plane on which they are drawn. A series of very interest- 

 ing results however are obtained by uniting the images of lines 

 meeting at an angular point, when the eye is placed at differ- 

 ent heights above the plane of the paper, and at different 

 distances from the angular point. 



Let AC, BC 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. Let R 

 be the right eye and L the left eye, and when at E'" let them 



• This effect is finely seen in the tliagram of the lloinogeneons Curve, 

 which forms plate 9. of Mr. Hay's work " On the Harmony of Form." 



