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THE MOVING PICTURE WORLD. "7 Piano- convex Plano-concave Double-convex ■ ■ -' ..... Double-concave g I Meniscus F ^^^jgfllfffflfflililllittitllfilDmiBlB^^, Concavo-convex Fig. i. Objective lius of which, or half the diameter, is two inches or ty inches; or, in other words, were the portion of the lere on which it is ground formed into a globe of cor- ponding convexity, it would be four inches or eighty hes in diameter. [Tie axis of a lens is a straight line drawn through the iter of its spherical surface; and as the spherical es of every lens are arches or circles, the axis of the s would pass through the center of that circle of which sides are segments. Rays are those emanations of light ich proceed from a luminous body, or from a body t is illuminated. The Radiant is that body or object which emits the rays of light, whether it be a self- luminous body, or one that only reflects the rays of light. Rays may proceed from a Radiant in different directions. They may be either parallel, converging or diverging. Parallel rays are those which proceed equally distant from each other through their whole course. Rays proceeding from the sun, the planets, the stars, and distant terrestrial objects are considered as parallel. Converging rays are such as, proceeding from a body, approach nearer and nearer in their progress, tending to a certain point where they all unite. Thus, the rays proceeding from the object to the point, are said to converge towards'that point. All convex glasses cause parallel rays which fall upon them to converge, in a greater or less degree; and they render converging rays still more convergent. If A B represents a convex lens, and H G I parallel rays falling upon it, they will be refracted, and converge towards the point F, which is called the focus, or burning point; because when the sun's rays are thus converged to a point by a large lens, they set on fire 'combustible substances. In this point the rays meet and intersect each other. Diverging rays are those which proceed from any point. The following designs show the effects of parallel, converging, and di- verging rays, in passing through a double convex lens: The center design shows the effects of parallel rays, K A, D E, L B, falling on a convex glass, A B. The rays which fall near the extremities at A and B are bent or refracted towards G F, the focus, and center of.con- vexity. It will be observed that they are less refracted as they approach the center of the lens, and the central ray DEC, which is called the axis of the lens, and which passes through its center, suffers no refraction. The lower design exhibits the course of converging rays when ' . • DIVERGING RAYS