on Physiological Optics. 317 



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 combination selected. These 

 cards were viewed in sunlight, never binocularly, but always 

 separately and 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 images per- 

 ceived. Without allowing him ever to know whether his 

 visual judgments were right or wrong, these experiments were 

 repeated day after day, until the youth's own conclusions 

 were definitely formed by repeated interpretation of his retinal 

 sensations. His judgments were in the majority of cases cor- 

 rect, during the latter part of the time invariably so ; and by 

 spectral images alone he learned what should be the proper 

 arrangement of pictures to produce a binocular resultant that 

 was concave or convex at will. The cards with concentric 

 circular bands were then substituted ; and in like manner he 

 soon learned what kind of obliquity should be given the plane 

 of each card in order to produce a concave or convex spectral 

 binocular image immediately afterwards. His eyes were not 

 sufficiently trained to enable him to test the effect of varying 

 the tension in either ciliary or rectus muscles, nor was he able 

 to perceive duplication in any part of any binocular spectral 

 image. 



5. A pair of diagrams were constructed in such manner as 

 to show very plainly the binocular duplication of central parts 

 in the background when the foreground was regarded and the 

 gaze was monocularly directed to the centre of each in suc- 

 cession, with the usual precautions. The spectral image pre- 

 sented the appearance of relief. By an effort of special 

 attention the duplication of the background became percep- 

 tible; but at the same moment the appearance of relief was 

 lost. 



Results from the Experiments just described. 



These experiments, in conjunction with those made by the 

 light of the electric spark, show that in the new mode of ste- 

 reoscopy play of the eyes is by no means necessary, although 

 it constitutes an important aid in all cases where a clear visual 

 judgment is not attainable at the first glance. They show 

 also very conclusively that the conscious perception of double 

 images in the binocular field of view, on which so much stress 

 was laid by Sir David Brewster*, far from being conducive to 

 clearness of binocular perception, tends rather to interfere 



* Brewster, ' The Stereoscope,' p. 76 et seq. 



