﻿of Binocular Vision. 



133 



teresting, not only as a beautiful illustration of the laws of 

 binocular vision, but, I believe, as throwing some light on the 

 subject of adjustment, and also upon the difficult subject of the 

 horopter. 



If a plane surface checkered or otherwise figured in regular 

 pattern, such as an oil floor-cloth, a tessellated pavement, or a 

 papered wall, be placed before the eyes at the distance of several 

 feet and the optic axes be then voluntarily converged (the eyes 

 crossed) upon some point in space nearer than the surface, the 

 figures will of course be all seen double. If, now, the conver- 

 gence be steadily increased until two contiguous similar images, 

 one belonging to the right eye and one to the left, are made to 

 coincide perfectly, and the eyes be then held steadily in this po- 

 sition for some time, the patterned surface will be distinctly seen 

 in exquisite miniature, not at its proper distance, but between 

 the real object and the eye, at a distance depending upon the 

 interval between the centres of the contiguous similar figures of 

 the pattern. If the pattern be very regular, the illusion is com- 

 plete ; we actually seem to 

 be looking at a real object. 

 In this experiment the po- 

 sition of the eyes is such 

 that, of two contiguous si- 

 milar figures, the right eye 

 is directed toward the left 

 figure and the left eye to- 

 ward the right figure ; and 

 the image is seen at the 

 crossing of the visual lines. 

 Thus if one eye be directed 

 toward a (fig. 1) and the 

 other toward b, a perfect 

 image of these two figures 

 will be seen at a 1 . So also 

 b and c will be united and 

 seen at c' and a and d at d\ 

 and so on for all the figures 

 of the pattern. The dotted line d a ] c' will be the position of the 

 image-surface. The image thus obtained may be a little indi- 

 stinct at first, but it gradually grows perfectly clear. As soon 

 as the image is distinctly seen and the outlines of the figures 

 well defined, it may be retained without any difficulty; for we 

 seem to be looking at a real object, and therefore retain the ne- 

 cessary convergence of the optic axes with ease. The eyes may 

 now be turned in every direction, viewing this extensive image- 

 surface precisely as if it were a real surface. 



