on Physiological Ojrtics. 315 



strain of the internal rectus and ciliary muscles produces the 

 illusion that the object perceived is smaller and nearer. 



The above is a mere statement of facts. Any discussion 

 they may suggest is reserved for a future paragraph. 



The Binocular Union of Spectral Images. 



If a sharply defined object be momentarily illuminated by 

 the intense light of the electric spark, a positive after-image 

 is perceived and quickly followed by a negative image of short 

 duration. If the gaze be very steadily fixed upon one point 

 of an object that is strongly illuminated by the direct rays of 

 the sun, the eyes being at the same time protected from the 

 glare proceeding from surrounding objects, a negative after- 

 image is obtained that lasts several minutes. Since its exist- 

 ence is due to fatigue of the retina in certain parts while others 

 remain unfatigued, such an image appears always in the direc- 

 tion of the visual line, changing in apparent position with 

 every motion of the eye. 



The late Professor W. B. Rogers, of Boston, published in 

 1860* some experiments on the binocular union of after- 

 images from illuminated lines so arranged as to produce the 

 appearance of relief. Perspective after-images were likewise 

 obtained by Wheatstone and by Wundt ; but an objection to 

 conclusions drawn from such perceptions as these consists in 

 the fact that the observer knows what effects would result in 

 direct vision under the conditions imposed; indeed he simply 

 retains a subjective perception of what he has just seen binocu- 

 larly. It is difficult to determine how far the perception may 

 be due to imagination rather than to immediate retinal sensa- 

 tion. Professor Rogers succeeded in attaining perspective 

 after-images even when the luminous lines were regarded suc- 

 cessively instead of together; but thus far no one else seems to 

 have confirmed this result; and the experiment is still liable 

 to the objection that the visual judgment is warped by antici- 

 pation and association. Experiments therefore have lately 

 been made with a view to testing these results, and at the 

 same time to ascertain whether any modification w T ould be im- 

 posed 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 centre, so that a white 

 band from top to bottom on a dark background was inclined 

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



* Proceedings of the American Association for the Advancement of 

 Science, 1860, p. 187 et seq. 



