Now the painter can imitate the aërial perspective. He skillfully diminishes the brightness, dulls the sharpness of outline, and blues the tinge of all objects, in proportion to their supposed distance, so as to produce the effect of depth of air. He can also and still more perfectly imitate the mathematical perspective, by diminishing the size of objects and the distance between them as he passes from his foreground to his background. But he can not imitate the focal perspective, and still less can he imitate the binocular perspective. This is artificially given only in the stereoscope, and is the glory of this little instrument. Focal perspective is unimportant to the painter, because imperceptible at the distance at which pictures are usually viewed; but the want of binocular perspective in painting interferes seriously with the completeness of the illusion. Therefore the illusion is more complete and the perspective comes out more distinctly when we look with only one eye. In a natural scene it is exactly the opposite: the perspective is far more perfect with both eyes open, because then all the forms coöperate.

Return to the Comparison of the Eye and the Camera. -It is time now to return to, and to continue, our comparison of the eye and the photographic camera. We have seen that both the camera and the eye are equally optical instruments contrived for the purpose of making an image; but we have also seen that in both this image is only a means by which to attain a higher end, viz., to make a photographic picture in the one case, and to accomplish distinct vision in the other. In both also, in order to accomplish its higher purpose, there must

be a sensitive receiving plate, viz., the iodized silver plate in the one, and the living retina in the other. In both, finally, there are wonderful changes, chemical or molecular or both, in the sensitive plate. Let us then continue the comparison.

1. In the photographic camera when accomplishing its work there are three images which may be mentally separated and described. First, the light-image. This is what we see on the ground-glass plate. It comes and goes with the object in front. It is the facsimile in form and color of the object, but diminished in size and inverted in position. Second, the invisible image. When the ground-glass plate is withdrawn and the sensitive plate substituted, the light-image falling on this plate determines in it wonderful molecular changes, which are graduated in intensity exactly according to the intensity and kind of light in the light-image: the aggregate effect is therefore rightly called an image, though it is invisible. Third, the visible image, or picture. The operator then takes the plate with the invisible image to a dark room, and applies certain chemicals which develop the image-i. e., which determine certain permanent chemical changes, which in intensity and kind are exactly proportioned to the antecedent molecular changes, and therefore graduated over the surface exactly as the molecular changes of the invisible image were graduated, and hence also exactly as the light of the light-image was graduated. This is the permanent photographic picture—the facsimile in form of the object which produced it.

So also in the work of the eye, vision, we may mentally separate and may describe three corresponding images. First, there is the light-image, which is formed in the dead as well as the living eye, and which comes

ent from other kinds, may be clearly demonstrated by the phenomena of inverse perspective now about to be described.

If stereoscopic diagrams suitably mounted for viewing in a stereoscope be combined with the naked eye by squinting (crossing the optic axes), as in Fig. 55 (page 153), or if such diagrams properly mounted for combination by squinting be viewed in the stereoscope, the perspective is completely reversed, the background becoming the foreground, and vice versa. For example, Fig. 56 represents a stereoscopic card. When the two pictures are combined with a stereoscope the result is a jelly-mold with the small end toward the observer; but if the same be combined with the naked eye by squinting, we have now beautifully shown the same jelly-mold reversed, and we are looking into the hollow. If there should be other forms of perspective strongly marked in the pictures, these may even be overborne by the inverse binocular perspective. For example, in the stereoscopic picture Fig. 57, representing the interior of a bridgeway, the diminishing size of the arches and the converging lines, even without the stereoscope, at once by mathematical perspective suggest the interior of a long archway. This impression is greatly strengthened by viewing it in the stereoscope; for the binocular perspective and the mathematical perspective strengthen each other, and the illusion is complete. But if we combine these with the naked eyes by squinting, we see with perfect distinctness, not a long hollow archway, the small arch representing the farther end, but a short conical solid, with the small end toward the observer. Thus the binocular perspective entirely overbears the mathematical.

The cause of this reversal of the natural perspective

FIG. 57.

b and b' in the background are nearer together than the points a and a' in the foreground. By combining these in a stereoscope, the background is seen nearer the ob

server at B, and the foreground thrown farther back to A. In Fig. 59 the pictures are mounted suitably for viewing in the stereoscope, but are combined by the naked eye. Here also the perspective is reversed,

for the background is seen at a nearer point B, and the foreground at a farther point A.

This inverse perspective is easily brought out, not only in stereoscopic diagrams, but in nearly all stereoscopic pictures, even in those representing extensive and complex views. In these, of course, when viewed in the stereoscope, the binocular is in harmony with other forms of perspective, and each enhances the effect of the other. But if we combine with the naked eyes by squinting, or if we reverse the mounting and view again