18 REPORT 1860. 



3. Without resorting to these troublesome efforts of subjective vision, the fol- 

 lowing experiment furnishes, as I think, conclusive proof that pictures successively 

 impressed on the respective eyes are sufficient for the stereoscopic effect. 



Let an opake screen be made to vibrate or revolve somewhat rapidly between 

 the eyes and the twin pictures of a stereoscopic drawing, so as alternately to expose 

 and cover each, while it completely excludes the simultaneous vision of any parts 

 of the two. The stereoscopic relief will be as apparent in these conditions as when the 

 moving screen is withdrawn. Here at each moment the actual impression in the 

 one eye and the retained impression in the other, form the elements of the per- 

 spective resultant perceived. 



It seems clearly inferrible from these experiments, that the perception of the 

 residtant in its proper relief does not require that each pair of corresponding points 

 should be combined by directing the optic axes to them pair by pair in succession, 

 as has been maintained. Nor is it necessary for the singleness of the resultant 

 perception, that the images of corresponding points of the object should fall on 

 what are called corresponding points of the retinse. The condition of single vision 

 in such cases seems to be simply this, that the pictures in the two eyes shall be such 

 and so placed as to be identical with the pictures which the real object would form if 

 placed at a given distance and in a given attitude before the eyes. 



4. I have of late years frequently repeated Dove's experiments with instantaneous 

 illumination, leading, as is well known, to similar conclusions. In these I have found 

 it most convenient to use the momentary bright flash of the Leyden bottle, con- 

 nected with the Ruhmkorft" coil according to Grove's plan. With a powerful 

 coil of Ritchie's construction, and a brass disc 8 inches in diameter having the usual 

 concentric striation, I am able, even with a single flash, to see the luminous line in 

 perspective, and by a quick succession of flashes, I can have it as steadily before 

 me as if illuminated by the sun. 



A twin-drawing of a simple geometrical solid, placed in the stereoscope, and 

 illuminated by the same means, appears single and in just relief in all cases where 

 the flashes recur at short intervals, and very frequently presents the same appear- 

 ance even with a single momentary light. 



To be assured that the effect was not due to the recollection of a previous stereo- 

 scopic impression, I have caused slides to be introduced, of which the form could 

 not be thus anticipated, and still have had no difficulty in describing the perspec- 

 tive residtant as exhibited by the instantaneous illumination. 



5. On the inability of the eyes to determine which retina is impressed. — Let a 

 small disc of white paper be fastened on a slip of black pasteboard of the size 

 of a stereoscopic slide, and let this be so placed in the instrument as to bring the 

 disc centrally in front of oue of the glasses, the person who is to view it being kept 

 in ignorance of the position of the spot. On looking into the instrument he will 

 think he sees it with both eyes equally, and, without resorting to the expedient of 

 closing his eyes .alternately, will be entirely unable to determine whether the spot is 

 before his right eye or his left eye. The spot appears to be placed in the mesial or 

 binocular direction, and in the same position as that of the resultant image of two 

 such discs, presented severally to the two eyes. 



It may be concluded from this that the mere retinal impression on either eye is 

 unaccompanied by any conscious reference to the special surface impressed, and that 

 the visual perception belongs to that part of the optical apparatus near or within 

 the brain, which belongs in common to both eyes. 



This experiment is moreover interesting from its bearing on the law of visible 

 direction. It shows that the sense of direction is just as truly normal to the central 

 part of the retina that has received no light from the object, as to the part of the 

 other retina upon which the white spot has been actually painted by the rays. In 

 truth it is normal to neither, but is felt to be in the middle line between the two, 

 that is, in the binocular direction. This experiment therefore contradicts the law, 

 which assumes that the direction in which an object appeal's is always in the normal 

 to the point of the retina impressed. 



