By the following simple method, the legend or inscription upon a coin may be read in absolute darkness. Polish the surface of any silver coin as highly as possible; touch the raised parts with aqua-fortis, so as to make them rough, taking care that the parts not raised retain their polish. Place the coin thus prepared upon red-hot iron, remove it into a dark room, and the figure and inscription will become more luminous than the rest, and may be distinctly seen and read by the spectator. If the lower parts of the coin be roughened with the acid, and the raised parts be polished, the effect will be reversed, and the figure and inscription will appear dark, or black upon a light or white ground. This experiment will be more surprising if made with an old coin, from which the figure and inscription have been obliterated; for, when the coin is placed upon the red-hot iron, the figure and inscription may be distinctly read upon a surface which had hitherto appeared blank. This experiment may be made with small coins upon a heated poker, a flat iron, or a salamander. The effect will be more perfect if the red-hot iron be concealed from the eye of the spectator: this may be done by placing upon the iron a piece of blackened tin, with a hole cut out, the size of the coin to be heated. TO MAKE A PRISM. Provide two small pieces of window-glass and a lump of wax. Soften and mould the wax, stick the two pieces of glass upon it, so that they meet, as in the cut, where _w_ is the wax, _g_ and _g_ the glasses stuck to it, (Fig. 1.) The end view (Fig. 2) will show the angle, _a_, at which the pieces of glass meet; into which angle put a drop of water. [Illustration: Fig. 1.] [Illustration: Fig. 2.] To use the instrument thus made, make a small hole, or a narrow horizontal slit, so that you can see the sky through it, when you stand at some distance from it in the room. Or a piece of pasteboard placed in the upper part of the window-sash, with a slit cut in it, will serve the purpose of the hole in the shutter. The slit should be about one-tenth of an inch wide, and an inch or two long, with even edges. Then hold the prism in your hand, place it close to your eye, and look through the drop of water, when you will see a beautiful train of colours, called a spectrum; at one end red, at the other violet, and in the middle yellowish green. [Illustration: Fig. 3.] The annexed figure will better explain the direction in which to look: here, _e_, is the eye of the spectator, _p_, is the prism, _h_, the hole in the shutter or pasteboard, _s_, the spectrum. By a little practice, you will soon become accustomed to look in the right direction, and will see the colours very bright and distinct. By means of this simple contrivance, white light may be analysed and proved to consist of coloured rays, and several of its properties be beautifully illustrated. OPTICAL AUGMENTATION.