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By our new method, on the contrary, all the phenomena are represented. In Fig. 116 is shown the visual result when the eyes are fixed on the background; in Fig. 117, the visual result when the eyes are fixed
on the foreground. In Fig. 116 we see that the nose n n' and the median screen ms m's are doubled heteronymously, and the space between the two is the common and only field of view (for the monocular fields
are cut off by the screen). In the middle between these is the binocular eye E, looking straight forward. This is manifestly exactly what we see in the stereoscope. Again, we see that the two images of the card have slidden over each other, in such wise that 6 b, Fig. 115, are brought together in the middle, united, and seen single in Fig. 116. But where? at what distance? Evidently this can only be at the point of sight, which, as I have already explained, is, in diagrammatic representations of visual phenomena, where the common visual line and the two median lines meet one another at the point B, Fig. 116. Meanwhile a a, Fig. 115, will have crossed over and become heteronymous, and their double images a a', Fig. 116, will be seen just where their ray-lines E a and E a' cut the median planes, viz., at a a'. In Fig. 117, which is the visual result when the eyes are fixed on the foreground, the shifting or sliding of the two images of the card is not quite so great as before. It is only enough to bring together the nearer points a a, Fig. 115, but not 6 b. These latter, therefore, are homonymously double. The united images of a a are seen single on the common visual line, and at the distance A where the double images of the median line cross each other; while 6 b are seen homonymously double, and at 6 b', the intersection of their ray-lines with the continuation of the median lines after crossing; for homonymous images are always referred beyond the point of sight.
The mode of representing combinations with the naked eyes by squinting is similar. Of course the place of the combined picture will in this case be between the eyes and the card. I reproduce (Fig. 118), for the
sake of comparison, the usual mode of representation * from page 139. In order to make the perspective nat
ural, it is necessary, as already explained, to reverse the mounting. In Fig. 118 the mounting is thus reversed, as seen by the fact that points in the foreground, a a, are farther apart than in the background, 6 b. The
usual mode of representation is shown in this figure. The true visual result is shown in Figs. 119 and 120, of which Fig. 119 represents the result when the observer is regarding the background, and Fig. 120 when he is regarding the foreground. It is seen that not mua
only does the diagram give truly the place and distance of the combined image, but also of the double images by means of which perspective is perceived.
It will be remembered that double images may be nearer or farther off than the point of sight, but that in the former case they are heteronymous, in the latter homonymous. In this way we at once perceive their distance in relation to point of sight. Now, in the new mode of representation, this fact is also indicated. In both of the figures 119 and 120 there are two places where the ray-lines cut the median lines, and therefore where double images may be formed; but in the one case the images are heteronymous, and therefore we refer them to the nearer points a a'; in the other case they are homonymous, and therefore we refer them to the farther points b b'.
If stereoscopic pictures mounted in the usual way be combined with the naked eyes by squinting, or pictures with reverse mounting be combined in the stereoscope, the perspective will be inverted. In this case the diagrammatic representation is exactly the same, except that the double images of points in the foreground a a' will now be homonymous, and therefore referred to the other possible point of reference, viz., beyond the point of sight; and double images of points in the background 6 b' will become heteronymous, and therefore referred to the nearer point. Some curious Phenomena illustrating the heteronymous
Shifting of the two Fields of View. Experiment 1.-To trace a picture where it is not. Take a postage stamp, or a piece of coin, or a medallion, or a small object or picture of any kind; place it on a sheet of white paper. Take then a thin opaque screen,
like a pamphlet, or thin book, or piece of cardboard, and set it upright on the right side of the object or picture, and bring down the face upon the top edge of the screen, in such wise that the latter shall occupy the median plane. If we now gaze with the eyes paralleli. e., on vacancy—the median card will double and become two parallel cards, and in the middle between them will be seen the object or picture. With a pencil in the right hand we may now trace the outline of the object or picture, by means of its image, on the right side of the screen, although the actual object or picture is on the left side of the same.
The accompanying diagrams illustrate and explain the phenomena. . In Fig. 121, R and L are the two eyes looking down on the paper sheet sh; ms is the median screen, and c the coin on its left side ; a, the spot where the outline is traced with the pencil P. This
figure therefore gives the actual condition of things. The visual result, and therefore the explanation, is given in Fig. 122. By careful inspection it is seen that the screen is doubled heteronymously, and becomes two parallel screens ms, m's; that the two images of the