Binocular Union of Spectral Images. 333 



shutting one eye, the component image that remained visible 

 to the other was at once projected upon the wall as a flat 

 picture. Strongly contracting the ciliary muscle of the eye 

 renaming open, without sensibly contracting the rectus mus- 

 cles, the picture was made to approach and grow apparently 

 Bmaller, in almost as marked a degree as by the previous exper- 



3. A series of concentric black and white circular bands was 

 constructed on a card, which was held in a vertical plane 

 obliquely crossing the horizontal visual line of the left eye. 

 After the retina became fatigued, the card was held across that of 

 the right eye, but with opposite obliquity, so that the distortions 

 of the elliptic images on the two retinas should be opposite in 

 sense. The resultant spectral image was concave instead of 

 plane, and presented the same variations with change of mus- 

 cular conditions as in previous experiments. 



4. To ascertain whether these perspective stereoscopic effects 

 were due to imagination and association, or whether they were 

 the immediate outcome of retinal sensation, from the existence 



atigue in the two eyes, 

 it was necessary to test some person whose eyes were normal, 

 but who was ignorant regarding the nature of 'the visual effects 

 to be produced, and who therefore could not be influenced by 

 n. I enlisted the interest of a young friend, a youth 

 of good general intelligence, but who was 'entirely unacquainted 

 with even the elementary principles of binocular vision. He 

 submitted to be trained until he could secure monocular after- 

 images successfully with either eye at will. Without granting 

 him the slightest clue by which results could be anticipated, I 

 employed a pair of cards on which were diagrams, so arranged 

 that the binocular resultant could be made either a raised cone, 

 a flat picture, or a hollow cone, according to the mode of com- 

 peted. These cards were viewed in sunlight, never 

 ohiocularly but always in succession, the relation between the 

 pictures being varied" in successive experiments. As soon as 

 the retinas were fatigued the observer was led into a perfectly 

 dark room and requested to describe the resultant spectral im- 

 ages perceived. Without allowing him ever to know whether 

 ms visual judgments were right or wrong, I repeated these 

 experiments day after day, until his own conclusions were 

 formed by repeated interpretation of his retinal sensations. 

 -His judgments were in the majority of cases correct, and by 

 spectral images alone he learned what was the proper arrange- 

 ment of pictures to produce a binocular resultant that was 

 concave or convex at will. I substituted then the cards with 

 concentric circular bands; and in like manner he soon learned 

 what kind of obliquity should be given the plane of each card 



