314 Mr. W. Le Conte Stevens's Notes 



ment of images which would have corresponded retinally if 

 the angle of rotation, <£, were zero. The attention being spe- 

 cially given to the centre of the binocular concave or convex 

 resultant, the illusion of binocular unity and depth in the pic- 

 ture remained possible when the retinal displacement cor- 

 responding to marginal portions of the combined image was 

 as great as "39 millim., or more than 80 times the diameter 

 corresponding to what has been estimated to be the minimum 

 visibile. By giving attention, through indirect vision, to the 

 marginal portions, the illusion of binocular unity was easily 

 destroyed, and double images at once became detectable. The 

 result was confusion and loss of the third dimension in space 

 at these marginal portions, while the perception remained clear 

 for central portions where no duplication could be perceived. 

 These effects were noticed by both Mr. Share and the writer. 



The pictures found best in these experiments were concen- 

 tric circles consisting of broad black bands on a white ground, 

 or of white bands on a black ground. Various other stereo- 

 graphs were employed, many of them constructed for the pro- 

 duction of stereoscopic relief, which could be reversed or 

 totally suppressed by appropriate arrangement of the cards on 

 the arms of the stereoscope. The peculiar nature of* the relief, 

 whether direct or reversed, was what the observer was re- 

 quested to ascertain, and with satisfactory results, usually 

 without delay. The most difficult case was that in which one 

 picture consisted of a red diagram on a green ground, the 

 other a green diagram of the same size on a red ground. 



A series of experiments, continued through many days, was 

 tried under illumination with the electric spark, by Mr. Share 

 and the writer jointly, to test still further the effect of mus- 

 cular strain in modifying the unconscious interpretation of the 

 binocular retinal image as discussed in a former paperf. The 

 optic angle was varied from 3° of divergence to 50° of con- 

 vergence of visual lines, while the stereograph of the moon 

 was again employed, being kept at a fixed distance on the 

 arms of the stereoscope while the observer, under the abnor- 

 mal conditions imposed, was requested to form an estimate of 

 apparent distance and diameter. Each acted as manipulator 

 and recorder for the other, the observer being kept ignorant 

 of his own record until the whole series of experiments was 

 completed. The result was in each case quite similar to that 

 formerly obtained with vision by continuous light; but the 

 limit of error was much wider, showing that under such un- 

 usual conditions no single visual judgment is worthy of any 

 confidence. The general effect on each, however, was that 

 * Philosophical Magazine, December 1881. 



