Fig. 239.
349. Lenses.—Transparent bodies having curved surfaces are called lenses. There are six kinds, represented in Fig. 238. The lenses in most common use are the double convex and double concave. The explanation of the mode in which these act upon light will sufficiently illustrate the operation of the others. They act by refraction, the convex collecting the rays, or bringing them nearer together, and the concave putting them farther apart. You can at once see, then, that a convex lens by causing the rays coming from an object to converge more, increases the visual angle, and therefore makes the object to appear larger than it otherwise would. This effect is illustrated by Fig. 239. The rays of light coming from the arrow are made by the lens so to converge as to meet at a, instead of b, where they would meet if they did not pass through the lens. That is, by passing through the lens they have a larger visual angle, and therefore the object is magnified. The distance between, c and d shows the size which the arrow would appear to have to the eye placed at a.
Fig. 240.
350. Microscopes and Telescopes.—What has been said of the action of the convex lens upon the visual angle will serve to explain the operation of the microscope. This instrument may be single or compound. The compound microscope has more than one lens, and is used to magnify very minute objects. Its operation may be seen by the diagram, Fig. 240. Rays from the object, E F, passing through the first lens, or object-glass, as it is called, form a magnified inverted image, G H, which is still more magnified by the eye-glass, C D. In the telescope we have also convex lenses, but they are arranged differently from those of the microscope, as the objects to be magnified are distant.
Fig. 241.
351. Magic Lantern.—This is an instrument by which pictures made upon slips of glass with coloring substances which allow the light to pass readily are thrown upon a screen magnified. It is a metallic lantern, A A, Fig. 241, with a concave reflector, p q, and two convex lenses, m and n. At c d is a space between the lenses into which the pictures are introduced. L is a strong light, which is in the focus both of the mirror and the lens m. The picture is therefore illuminated strongly by the rays reflected from the mirror and passed through the lens. The lens n which is movable, is so adjusted as to throw a highly magnified image of the picture upon the screen. As the image is an inverted one the pictures must be inserted upside down, that the images on the screen may be upright. The solar microscope is, in its essential parts, like the magic lantern, the sun being used as the illuminator.
Fig. 242.