NATURAL SCIENCES OP PHILADELPHIA. 99 



spectrum, and then directing the eyes towards a distant wall, a single spec- 

 trum was observed, having the attitude and relief of the original binocular 

 resultant. When the luminous lines were regarded in succession, each by the 

 corresponding eye, the other eye being shaded, so that no direct binocular 

 combination could be formed, it was found on looking towards the wall that 

 the subjective images united into a single spectral line, having the same relief 

 as if the lines had been directly combined in the stereoscope. 



In these experiments, according to the theory of Brewster, the resultant 

 spectrum, instead of being a single line in a perspective attitude ought to pre- 

 sent the form of two lines inclined or crossing, situated in the plane of the 

 wall without projection or relief. The conditions of the experiments are such 

 as exclude all opportunity of a shifting of the image on the retina, and this is 

 essential to the successive combinations of pairs of points required by the theory 

 in the production of perspective effect. 



A similar result was still more clearly shown by vibrating a screen between 

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

 and cover each, completely excluding the simultaneous vision of the two. 

 The stereoscopic relief was as apparent in these conditions as when the vibra- 

 ting screen was withdrawn. 



The perception of the resultant in its proper relief does not therefore require 

 that each pair of corresponding points should be combined by directing the 

 optic axes to them pair by pair in succession, as maintained by Brewster. 

 Nor is it necessary for the singleness of the resultant perception that the 

 images of corresponding points of the objects 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. 



Dr. Ruschenberger asked how it is, under the explanation given by Prof. 

 Rogers, that a man with only one eye is capable of perceiving solidity, and of 

 appreciating the properties of photographs viewed stereoscopically. 



Mr. Powel asked at what rate per second the vibrating or revolving screen 

 presented its openings ; for if it happened eight or ten times in a second, 

 might it not fail to practically intercept vision ? Objects thus seen would ap- 

 pear permanently. Thus, although not appearing to each eye at the identical 

 instant of time, the object would be persistent in both, for an impression upon 

 the eye cannot be discharged oftener than about eight times in a second, some 

 impressions remain much longer. An object illuminated by a flash of light- 

 ning for a very instant, may thus appear solid to both eyes, the intense re- 

 flection impressed upon the retina endures long enough for the sensorium to 

 scan it in detail. A man takes quick aim with a rifle, it may be almost in- 

 stantaneously, yet by distinct operations and different foci of vision he must 

 see the distant mark the tip sight, and again the heel sight, no two of which 

 can be in focus at once. We have here successive points in a line, rapidly 

 scanned in determining position. The breadth of field of distinct vision is 

 exceedingly narrow for the same instant of time, and so is the penetration of 

 focus very short. A separate direction and a new adjustment of the eye must 

 be given for parts of even a very small object. 



Mr. P. remarked, while upon the subject, that he believed the stereoscopic 

 effect often noticed in viewing large photographic pictures with only one eye, 

 was caused by the aperture of the lens used in taking the picture ; for the 

 aperture is often so great that objects have an appreciable parallax from the 

 opposite margins of the aperture, and the picture thus contains more than 

 could be seen from one point. When both eyes, however, view such a picture 

 they decide that it is flat and in one plane, and their evidence denies the 

 stereoscopic effect which one eye cannot so well dispute. 



I860.] 



