316 Mr. W. Le Conte Stevens's Notes 



rately in succession, while held in direct sunlight, until both 

 retinas were fatigued. On going then into a slightly darkened 

 room, the inclined spectral image was easily perceived, appa- 

 rently in mid-air. On making the visual lines parallel, it 

 became projected on the wall, but without losing its obliquity. 

 On strongly contracting the internal rectus muscles, it ap- 

 peared still directly in front, but much smaller and nearer. 

 The experiment was repeated many times, and varied, but with 

 uniform results. 



2. On separate cards a pair of diagrams were constructed 

 in such a manner as to produce an image in relief when bin- 

 ocularly view T ed, in the stereoscope or otherwise. These were 

 separately and successively regarded in sunlight, each with 

 the appropriate eye. In the dark room the resultant after- 

 image appeared in mid-air in clear relief. On shutting one 

 eye, the component image that remained visible to the other 

 was at once projected upon the Avail as a flat picture. Strongly 

 contracting the ciliary muscle of the eye remaining open, with- 

 out sensibly contracting the rectus muscles, the picture was 

 made to approach and grow apparently smaller, in almost as 

 marked a degree as by the previous experiment. 



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 had become fatigued, the same 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. Each eye was closed while the 

 other was receiving light from the card. The resultant spec- 

 tral image was concave instead of plane, and presented the 

 same variations with change of muscular conditions as in pre- 

 vious 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 

 of dissimilar images remaining through fatigue in the two 

 eyes, it was necessary to test some one 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 

 anticipation. It was found possible to enlist the interest of a 

 youth of good general intelligence, who was entirely unac- 

 quainted 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, the writer employed a pair of cards on 



