upper base. When the latter is made an object of attention, two dots are seen on the lower base, which apparently broadens out on the two sides; but the moment the attention is concentrated upon them, unless the observer is a little skilled in indirect vision, they fuse into one, and four instead of two projections are seen at the upper base, which in turn has broadened out. Indeed, very little skill is needed to perceive the distinct duplication of the entire upper base. At the middle of the field of view is an inclined black parallelogram, on which no duplication of any part can be perceived, except by a very steady gaze or by comparison with the black circles above and below; and these in turn are made to appear at different distances from the observer. The circular arc and straight line, each marked a, combine into a distorted parabola, in which the concavity is perfect at the middle, but at the top and bottom it breaks into two separate lines. The resultant of the lines marked b is quite as distinctly curved, but many persons will fail to notice any duplication at all; and this remark applies still more forcibly to the resultant c. The group d forms a warped surface; but, if the resultant line at its right be fixedly examined, it will be seen as an oblique cross, the combination being effected only by motion of the eyes. The group e presents still greater difficulties. The pair f are nearly horizontal, and are coalescent at the middle, but not combined at the extremities; each component hence appears no longer straight. The arrows at the left point obliquely, some toward and some from the neighborhood of the observer; but, if the gaze be rigidly directed to the vertical rod on which they are fixed, a pair of well-practiced eyes will perceive some of them to have very mobile double heads. The circles x, y, and z have a common axis, and are successively nearer to the observer; x and z are highly lustrous, and, when either is regarded separately, y is by indirect vision seen slightly double.
The two halves of this stereograph are strikingly dissimilar, but the principle which it illustrates enables us to secure stereoscopy with a pair of absolutely similar figures by so adjusting these in position that advantage may be taken of the almost spherical surface of the back of each eye. The geometric explanation of this is unsuited to the present article.[1] It may be sufficient to state that, if the two pictures be oppositely inclined to the visual lines, instead of being directly across these, the retinal images must be dissimilar, and the subjective combination of these must hence present the appearance of relief, which may be varied at will by varying the inclination of the cards.
The theory of associated muscular action which has been illustrated, while undoubtedly true, is still not sufficient by itself to explain all the phenomena of stereoscopy. The perception of distinct relief is possible when the card is illuminated with the electric spark. No motion of the eyes is attainable during so minute an interval. It is
- ↑ See "American journal of Science" for April, 1882, p. 297, and May, 1882, p. 359.
