DISTANCE GIVEN BY BINOCULAR VISION. (J71 



binocular union of similar images. This has, no doubt, arisen from the rare oc- 

 currence of these circumstances or conditions, under which alone such illusions 

 can be produced. In a room where the paper hangings have a small pattern, or 

 similar figures recurring at the distance of 1, 1^, or 2 inches, a short-sighted per- 

 son might very readily turn his eyes on the wall, when then* axes converged to 

 some point between him and the wall, which would unite one pair of the similar 

 images ; and, in this case, he would see the wall nearer him than the real wall, 

 and moving with the motion of his head like something aerial. In like manner, 

 a long-sighted person, with his optical axes converged to a point beyond the wall, 

 might see an image of the wall more distant, and of an aerial character ; or a 

 person who has taken too much wine, which often fixes the optical axes in oppo- 

 sition to the will, might, according to the nature of his sight, witness either of the 

 illusions above mentioned. 



In the preceding observations, we have confined ourselves to the binocular 

 union of figures upon an opaque ground. This limitation almost necessarily pre- 

 cluded us from observing the results when the binocular centre is beyond the 

 plane where these figures are situated, because it is not easy to adjust the eyes to 

 a distant object, unless we look through the surfaces containing the figures. Now, 

 this is by far the most interesting form of the experiment, and it has the advan- 

 tage of putting scarcely any strain upon the eyes, not only because the binocular 

 centre is more distant, but because we cannot, in this way, unite figures whose 

 distance exceeds 2^ inches, the interval between the eyes. Transparent patterns 

 for these experiments may be cut out of stiff card paper, or thin plates of metal, 

 or they may be made of paper pasted upon large panes of glass. Experiments 

 may be made with trellis work, or with windows composed of small squares or 

 lozenges ; but the readiest pattern is the cane bottom of a chair, and I have per- 

 formed my experiments by simply placing such a chair upon a high table, with 

 its cane bottom in a vertical position. The distance of the centres of the eight- 

 sided open figures in the direction of the width or depth of the chair, varies in 

 different patterns from 0.54 to 076 of an inch. In order to simplify the calcula- 

 tions, we shall take the distance at 0-5, or half an inch. Then let 

 D = 12 inches be the distance of the pattern from the eyes. 

 d = 0*5 the distance of the centres of the similar figures. 



+ A = distance of suspended image from, and in front of, the pattern. 



A' = distance of suspended image from, and behind, the pattern. 

 C = 2'5 the distance between the eyes. 



Then we shall have 



Dd Dd 



+ A =: ~^ and ~ A == ' Hence 



D A = distance of suspended image from the eye, and in front of the pat- 



tern, and 

 D + A' = its distance from the eye, and behind the pattern. 



VOL. XV. PART IV. 8 T 



