picture. The two views are painted on opposite sides of a vertical screen. The first effect is painted upon the front of the screen, and is seen by light that enters a window, M, and falling upon a movable mirror, E, is thrown so as to illuminate the front of the screen. The room behind the screen being dark, no part of the picture on the other side of the screen is seen.

If, now, the mirror E, be lowered gently, the shutters, NN, being at the same time slowly opened, the picture on the front of the screen will fade away, to be replaced by that on the other side, now seen by transmitted light. When the mirror is let completely down, and the shutters, NN, are completely opened, the only effect that will be seen will be that from behind.

The diorama was invented and perfected by DAGUERRE, the celebrated discoverer of the daguerreotype. Many of his pictures of this kind had a high reputation, among which may be mentioned his Midnight Mass, and his Valley of Goldeau.

333.

The Camera Obscura.

The CAMERA OBSCURA is an instrument used for forming a clear picture of objects upon a screen of ground glass or paper.

It consists, Fig. 226, of a closed box mounted on a stand, having a small hole on one side and a screen for receiving the image on the opposite side. The hole may be of any dimensions, if a concave lens be placed in it capable of filling it, and of such power as to bring the rays to a focus on the opposite screen.

Fig. 226 shows how the image is formed in the camera obscura. The pencil of rays coming from the soldier's cap goes to form an image at the bottom of the box, whilst that coming from his feet goes to form an image at the top of

Explain the method of exhibiting. is the Camera Obscura? Describe it.

Who invented the diorama? (333.) What
Explain the course of the rays.

the screen.

Fig. 226.

The image is inverted and reversed in a horizontal direction, but in every other respect, including color, it is a perfect representation of the object pictured.

The camera obscura affords aid in sketching the outlines of a landscape or building, but its principal importance at present consists in its application to the various branches of Photography. It may also be used as a source of amusement.

The images formed by a camera obscura possess the remarkable peculiarity of being entirely independent of the shape of the opening in the box, provided it be quite small. The shape of the images is the same, whether the opening be square, round, triangular, or oblong.

To show this, let us consider the case of a beam of solar light entering a dark room through a hole in a shutter, Fig. 227. With respect to the sun, the hole in the shutter is but a point, hence the group of rays which enter it form in reality a cone whose base is the sun. The prolongation of these rays into the room makes up another

cone similar in shape to the first, and if this conė be intercepted by a screen perpendicular to the line joining the hole with the centre of the sun, the image formed will be a circle. If the rays are intercepted by an oblique plane, as in the figure, the image is elliptical, but it never takes the form of the hole when that is small.

In accordance with this principle, we find the illuminated patches of earth formed by light passing between the leaves in a forest of a circular or elliptical shape. This is illustrated in Fig. 228. In an eclipse of the sun, when the visible portion of the sun is of cresccut shape, the patches of light all assume the crescent form; that is,

they are images of the visible part of the sun. The reason of this curious phenomenon is evident.

Manner of rendering the Image erect.

334. The manner of producing erect images of external objects in a camera obscura, or dark room, is shown in Fig. 229. A little above the hole a plane mirror is so placed as to reflect the rays which enter it upon a convex lens fixed at the extremity of a tube. This reflection inverts the beam of light and makes the image erect, which may then be thrown upon a suitable screen for observation.

Such images are perfect representations of the external objects which they represent, being perfectly faithful, not only in form and color, but in motion also. When images of street scenes, with all their life and motion, are thus formed, they are very striking as well as interesting.