TRANSPARENT BODIES.
Transparent bodies, such as glass, may be made of such form as to cause all the rays which pass through them from any given point to meet in any other given point beyond them, or which will disperse them from the given point. These are called lenses, and have different names according to their form. 1. Is called the plano-convex lens. 2. Plano-concave. 3. Double convex. 4. Double concave. 5. A meniscus, so called from its resembling the crescent moon.
THE PRISM.
The prism is a triangular solid of glass, and by it the young optician may decompose a ray of light into its primitive and supplementary colours, for a ray of light is of a compound nature. By the prism the ray A is divided into its three primitive colours, blue, red, and yellow; and their four supplementary ones, violet, indigo, green, and orange. The best way to perform this experiment is to cut a small slit in a window-shutter, on which the sun shines at some period of the day, and directly opposite the hole place a prism P; a beam of light in passing through it will then be decomposed, and if let fall upon a sheet of white paper, or against a white wall, the seven colours of the rainbow will be observed.
COMPOSITION OF LIGHT.
The beam of light passing through the prism is decomposed, and the spaces occupied by the colours are in the following proportions:—red, 6; orange, 4; yellow, 7; green, 8; blue, 8; indigo, 6; violet, 11. Now, if you paste a sheet of white paper on a circular piece of board about six inches in diameter, and divide it with a pencil into fifty parts, and paint colours in them in the proportions given above, painting them dark in the centre parts, and gradually fainter at the edges, till they blend with the one adjoining. If the board be then fixed to an axle, and made to revolve quickly, the colours will no longer appear separate and distinct, but becoming gradually less visible they will ultimately appear white, giving this appearance to the whole surface of the paper.
