Single Vision with two Eyes.

635

two eyes are directed to the object at A, they form at meeting a large or squinting angle, indicated by the letters a A a': when they meet at B, the angle b B b' is smaller, or more acute: when they meet at C, the angle c C c′ is smaller still; and the different degrees of effort made to produce the necessary convergence, at the several points of which effort the person is as conscious as of that exerted to bring the hands together, to examine by touch an object in the dark, becomes a measure, to a certain degree, of the distances. There occur at the same time these other facts. While the eyes are directed to the pin at B, seeing it clearly and singly, by the rays indicated here by the strong lines, B b and B b', they are receiving also from the more distant pin at C, rays of light indicated by the dotted lines, C c and Cd; but the spots of the retina, c and c', give the sensation of two images formed on less sensitive parts of the retina, and they are scarcely noticed. Then, further, the eyes are admitting light also from the pin at A, nearer to them than the object at B, which light reaches the spots of the retina, a and a', on which two other indistinct images are formed. Thus, in any case of vision, the object on which the axes of the eyes meet is alone seen single and distinct, while any other objects at greater or less distances within the field of view, are seen double and indistinct. This is strikingly seen by looking along the rod and pins above de. scribed. It follows, of course, that the rod itself appears as two rods, except at the point supporting the object, to which the eyes are specially directed, and there the two appear to cross each other.

A still simpler experiment than that above described, is, to hold up a finger a few inches from the eyes, and while looking steadily at it, to attend to the more faint appearance of things farther off, or nearer, as a book, a picture, a candle. These all appear double. If two fingers be held up in a line, at different distances from the eyes, the one, looked at directly, is always single and clear, the other always double and indistinct. In reading printed or written characters, a person sees distinctly at one time only three or four letters, because these alone are sending light to a true focus in the centres of each retina; bit the other letters immediately around, although indistinct, do not appear double, because they are nearly at the same distance from the eyes as those well seen.

867. The Stereoscope.—In the last two paragraphs we have the means of explaining the singularly interesting invention of the late Professor Wheatstone, called by him the Stereoscope (σrepeos, solid;

636

Invention of the Stereoscope.

σкодεw, to see), another fruit of the same rare sagacity and exhaustless mechanical ingenuity which first devised and constructed for the world, the working Electric Telegraph.

As the eyes judge of the distance of larger objects around the person, by the degree of convergence of their axes, required to give perfect vision of these, so do they judge of the size and shape of single solid bodies, which may be regarded as collections of minute parts or points, joined together by the angles of convergence required to see clearly the relative distances of the different points.

A consideration of the following experiments brings the important particulars under review. In fig. 211, let No. 1 represent a solid

outline formed in wire of a small pyramid, of which the top is wanting, placed on a block, A B. If a person look directly down upon this with one eye, it will have the appearance of No. 2, with perfectly equal sides, and equal angles all round, and with the small

The Principle of the Stereoscope.

637

square, a b c d, in the middle of the large one, e f g h. If it be then placed midway before the eyes, that is, opposite to the middle of the face, the right eye alone looking at it, will see the sloping surface of the right side more directly than that of the left side, and the appearance to that eye will be, as shown in No. 3, the small square, a b c d, appearing no longer in the middle of the larger, but rearer to the bottom line, eg, on the left side. If, then, the left eye alone be directed to the pyramid, it will appear as represented in No. 4, with the small square near the line, ƒ h, on the right side. If a pane of glass, therefore, coated with gum, werc placed between the eyes and the models, Nos. 3 and 4, and the view for each eye were traced on the glass, the tracing would be an exact counterpart of what is shown on this paper. This explains that two eyes cannot receive exactly similar images on each retina, at the same time, from the same solid body. By comparison, however, of the different images, a judgment is quickly made of the true form and position of the body. The experiment described may be made at once by setting up on edge between the two figures, Nos. 3 and 4, equally exposed to the light, a large card or a thin volume, so that when the eyes approach to within six or eight inches, each can see only the one drawing before it. The eyes then, after a little practice, will see, not the two drawings, differing from each other, but a single very perfect representation of the pyramid of solid wire, of which the upper part will appear clearly to be at least an inch nearer to the eye than the bottom.

To say that two eyes viewing the same object have on their retinas images differing from each other, according to the laws of perspective, just as in two camera obscuras placed side by side, like the eyes, pictures of the same object differ—and that the images in the eyes coalesce in some way, so as to give to the person a clear perception of the solidity and shape of the object—to say this, is to state the fact, and not to explain it. If it be added, however, according to the details given in Art. 866, that the two eyes sce at one time, singly and clearly, so as to judge of its distance by the required degree of convergence of the optical axes, only the one point of the object then looked at, while the other points appear indistinct or double, and that by then changing quickly the direction and convergence of the axes to the other points, the distances and positions of all are noted; the required explanation is thus afforded.

868. For stereoscopic representations two pictures are required

638

Stereoscopic Pictures.

To draw these correctly, according to the principles of perspective for complex objects, such as a landscape or the human countenance, is so difficult, that if the camera obscura and photography, which give such drawings at once, had not been invented, the stereoscope would have been little used. The eyes find the two views of an object required, presented in the object itself, and therefore in the same place. The two views made for the stereoscope, are caused to be seen as if together or mingled, by mirrors or by lenses-the one is called the reflecting, the other the lenticular or refracting stereoscope. By reversing the position of stereoscopic pictures from right to left, the protuberant parts of objects appear depressions and vice versa. The prominent pyramid above sketched, if so used, will appear a hollow or of a cave-like shape. Such a change is called pseudoscopic (yevdns, false).

The stereoscope proves to us that each eye sees an object adapted to its own axis of vision, and that the perfect form or perspective of the object, results from the union of the two pictures by a mental operation. In the annexed figure, representing a section of a railway tunnel (fig. 212), it will be seen that the distant opening

of the tunnel, is to the left of the centre on the left side, A, and to the right on the right side, B, the perspective lines inclining accordingly. When viewed at about four inches from the page three engravings will appear, the central one being mentally compounded of the other two, and representing the distant opening and the perspective lines exactly in the centre. On shutting either eye, the central image will disappear, and two only will be seen as they are represented on the page.

Panoramic Vicws.

639

A similar effect is produced by coloured objects. In a stereoscopic drawing a table-cover, coloured red on one side and blue on the other, presents a shot colour, or mixture of the two, when viewed by both eyes, but red or blue only when seen by onc.

869. When a picture on a plane surface has to represent objects supposed far from the eye, the farther the picture itself is placed from the eye, supposing the figures to be made of a size duly proportioned, the more perfect will the illusion become, because the divergence of rays and convergence of the axes (two circumstances in which the effect of a mere picture on the eye must always differ from the effect of a real scene) will be more nearly what occurs in nature. This explains in part why the picture called panorama (from Greck words, signifying a view of the whole) is an exhibition so pleasing. The painting is removed to a considerable distance from the eye, and the near objects are drawn on a proportionately large scale, causing the eyes to feel that the light comes from a considerable distance, and that their axes do not need to approximate or converge much. When, in such a case, the first impression of the want of absolute conformity to nature has passed away, the illusion becomes nearly complete. Another important peculiarity in the panorama is, that instead of being a painting on a plane surface, like common pictures, of which the sides are more distant from the eyes than the centre, and which embraces only a small part of the whole sphere of view, it is on a curved concave surface entirely surrounding the spectator, and on which all the objects visible in various directions from the supposed point of view, are seen in the very situations which in nature they hold; and the spectator is enabled to conceive much more distinctly of each particular by seeing it in relation to the others around. Few persons can forget the vivid impressions of surprise and pleasure made on them by the first panorama which they visited; and after increased experience and more enlightened judgment, they will discover still additional reasons for admiring this marvellous mode of being instantly transported to any distance, to contemplate at leisure some interesting scene, represented under the most favourable circumstances of point of view, light, and weather.

It corrects slight remaining optical defects of a common panorama to view it through a large lens, of which the focal distance is equal to the distance of the picture from the eye. This has the effect of diminishing the divergence of the luminous rays until it