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. [Illustration: FIG. 4.]