Such is a brief history of the lenticular stereoscope, of its introduction into Paris and London, and of its application to portraiture and sculpture. It is now in general use over the whole world, and it has been estimated that upwards of half a million of these instruments have been sold. A Stereoscope Company has been established in London[30] for the manufacture and sale of the lenticular stereoscope, and for the production of binocular pictures for educational and other purposes. Photographers are now employed in every part of the globe in taking binocular pictures for the instrument,—among the ruins of Pompeii and Herculaneum—on the glaciers and in the valleys of Switzerland—among the public monuments in the Old and the New World—amid the shipping of our commercial harbours—in the museums of ancient and modern life—in the sacred precincts of the domestic circle—and among those scenes of the picturesque and the sublime which are so affectionately associated with the recollection of our early days, and amid which, even at the close of life, we renew, with loftier sentiments and nobler aspirations, the youth of our being, which, in the worlds of the future, is to be the commencement of a longer and a happier existence.
CHAPTER II.
ON MONOCULAR VISION, OR
VISION WITH ONE EYE.
In order to understand the theory and construction of the stereoscope we must be acquainted with the general structure of the eye, with the mode in which the images of visible objects are formed within it, and with the laws of vision by means of which we see those objects in the position which they occupy, that is, in the direction and at the distance at which they exist.
Every visible object radiates, or throws out in all directions, particles or rays of light, by means of which we see them either directly by the images formed in the eye, or indirectly by looking at images of them formed by their passing through a small hole, or through a lens placed in a dark room or camera, at the end of which is a piece of paper or ground-glass to receive the image.
In order to understand this let h be a very small pin-hole in a shutter or camera, mn, and let ryb be any object of different colours, the upper part, r, being red, the middle, y, yellow, and the lower part, b, blue. If a sheet of white paper, br, is placed behind the hole h, at the same distance as the object rb is before it, an image, br, will be formed of the same ray and the same colours as the object rb. As the particles or rays of light move in straight lines, a red ray from the middle part of r will pass through the hole h and illuminate the point r with red light. In like manner, rays from the middle points of y and b will pass through h and illuminate with yellow and blue light the points y and b. Every other point of the coloured spaces, r, y, and b, will, in the same manner, paint itself, as it were, on the paper, and produce a coloured image, byr, exactly the same in form and colour as the object ryb. If the hole h is sufficiently small no ray from any one point of the object will interfere with or mix with any other ray that falls upon the paper. If the paper is held at half the distance, at b′y′ for example, a coloured image, b′y′r′, of half the size, will be formed, and if we hold it at twice the distance, at b″r″ for example, a coloured image, b″y″r″, of twice the size, will be painted on the paper.
Fig. 4.
As the hole h is supposed to be so small as to receive only one ray from every point of the object, the images of the object, viz., br, b′r′, b″r″, will be very faint. By widening the hole h, so as to admit more rays from each luminous point of rb, the images would become brighter, but they would become at the same time indistinct, as the rays from one point of the object would mix with those from adjacent points, and at the boundaries of the colours r, y, and b, the one colour would obliterate the other. In order, therefore, to obtain sufficiently bright images of visible objects we must use lenses, which have the property of forming distinct images behind them, at a point called their focus. If we widen the hole h, and place in it a lens whose focus is at y, for an object at the same distance, hy, it will form a bright and distinct image, br, of the same size as the object rb. If we remove the lens, and place another in h, whose focus is at y′, for a distance hy, an image, b′r′, half of the size of rb, will be formed at that point; and if we substitute for this lens another, whose focus is at y″, a distinct image, b″r″, twice the size of the object, will be formed, the size of the image being always to that of the object as their respective distances from the hole or lens at h.
With the aid of these results, which any person may confirm by making the experiments, we shall easily understand how we see external objects by means of the images formed in the eye. The human eye, a section and a front view of which is shewn in [Fig. 5, a], is almost a sphere. Its outer membrane, abcde, or mno, [Fig. 5, b], consists of a tough substance, and is called the sclerotic coat, which forms the white of the eye, a, seen in the front view. The front part of the eyeball, cxd, which resembles a small watch-glass, is perfectly transparent, and is called the cornea. Behind it is the iris, cabe, or c in the front view, which is a circular disc, with a hole, ab, in its centre, called the pupil, or black of the eye. It is, as it were, the window of the eye, through which all the light from visible objects must pass. The iris has different colours in different persons, black, blue, or grey; and the pupil, ab, or h, has the power of contracting or enlarging its size according as the light which enters it is more or less bright. In sunlight it is very small, and in twilight its size is considerable. Behind the iris, and close to it, is a doubly convex lens, df, or ll in [Fig. 5, b], called the crystalline lens. It is more convex or round on the inner side, and it is suspended by the ciliary processes at lc, lc′, by which it is supposed to be moved towards and from h, in order to accommodate the eye to different distances, or obtain distinct vision at these distances. At the back of the eye is a thin pulpy transparent membrane, rr o rr, or vvv, called the retina, which, like the ground-glass of a camera obscura, receives the images of visible objects. This membrane is an expansion of the optic nerve o, or a in [Fig. 5, a], which passes to the brain, and, by a process of which we are ignorant, gives us vision of the objects whose images are formed on its expanded surface. The globular form of the eye is maintained by two fluids which fill it,—the aqueous humour, which lies between the crystalline lens and the cornea, and the vitreous humour, zz, which fills the back of the eye.
