W. LeConte Stevens — Physiological Optics: 



-asts sharply with the surrounding area, while the whole is 

 nnated by the direct beams of the sun, the eyes being at 

 ame time protected from any glare proceeding from other 

 tions. To secure these visual effects some ocular training 

 the perception soon becomes almost as vivid 

 nary vision. The after-image, being due to fatigue 

 of the retina in certain parts while others remain un fatigued, 

 appears in the direction of the visual line, changing in apparent 

 position with every motion of the eye. 



Professor W. B. Rogers* in 1860 published some experi- 

 ments in the binocular union of after-images from illuminated 

 lines so arranged as to produce the appearance of relief. Per- 

 spective after images were likewise obtained by Wheatstonef 

 and Wundt;J but the objection to conclusions drawn from such 

 perceptions as these consists in the ! < t that the observer knows 

 what effects would result in direct vision, under the conditions 

 imposed ; and it is difficult to determine how far the perception 

 may be due to imagination rather than to retinal sensation. 

 Professor Rogers succeeded in attaining perspective after-ima- 

 ages. even when the luminous lines wen- regarded successively 

 t together; but thus far no one else seems to have 

 recorded the same results, and the experiment is still liable to 

 the objection that the visual judgment is warped by Anticipa- 

 tion and association. I have undertaken to test these results 

 as rigidly as possible, and al the same time to ascertain whether 

 any modification would be imposed by varying the muscular 

 conditions under which the spectral images are seen. 



1. Across the median plane of vision was held a card, with 

 the upper edge more remote than the center, so that a white 

 band from top to bottom on a dark background, was inclined 

 about 40°. This was fixedly regarded with each eye in suc- 

 cession, while held in direct sunlight, until both retinas were 



noderately dark room, the inclined 

 ^ perceived, apparently in mid air. 

 r ith visual lines parallel it became projected on the wall but 

 without losing its obliquity. On strongly contracting the 

 internal rectus muscles, it appeared 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 manner as to produce an image in relief when binocularly 

 viewed in the stereoscope or otherwise. These were separately 

 and successively regarded in sunlight, each with the appropri- 

 ate eye. In the dark room the resultant a der-i mage was dis- 

 tinctly perceived in mid air. standing out in bold relief. On 



