He should also practice using the blowpipe until he can keep up a steady and uninterrupted flame for ten minutes or a quarter of an hour, without stopping for breath. It is quite possible to replenish _wind_ in the mouth, which alone ought to be used, without interrupting the breathing for an instant, but it requires some practice. HOW TO BECOME AN OPTICIAN. Optics is the science of _light_ and _vision_. Concerning the nature of light, two theories are at present very ably maintained by their respective advocates. One is termed the Newtonian theory, and the other the Huygenean. The Newtonian theory considers light to consist of inconceivably small bodies emanating from the sun, or any other luminous body. The Huygenean conceives it to consist in the undulations of a highly elastic and subtle fluid, propagated round luminous centers in spherical waves, like those arising in a placid lake when a stone is dropped into the water. LIGHT AS AN EFFECT. Light follows the same laws as gravity, and its intensity or degree decreases as the square of the distance from the luminous body increases. Thus, at the distance of two yards from a candle we shall have four times less light than we should have were we only one yard from it, and so on in the same proportion. REFRACTION. Bodies which suffer the rays of light to pass through them, such as air, water, or glass, are called refracting media. When rays of light enter these, they do not proceed in straight lines, but are said to be refracted, or bent out of their course. But if the ray falls perpendicularly on the glass, there is no refraction, and it proceeds in a direct line; hence, refraction only takes place when rays fall obliquely or aslant on the media. THE INVISIBLE COIN MADE VISIBLE. If a coin be placed in a basin, so that on standing at a certain distance it be just hid from the eye of an observer by the rim or edge of the basin, and then water be poured in by a second person, the first keeping his position; as the water rises the coin will become visible, and will appear to have moved from the side to the middle of the basin. THE MULTIPLYING GLASS. The multiplying glass is a semicircular piece of glass cut into facets or distinct surfaces; and in looking through it we have an illustration of the laws of refraction, for if a small object, such as a fly, be placed at the further end, a person will see as many flies as there are surfaces or facets on the glass. 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.