Page:Popular Science Monthly Volume 5.djvu/48

38 density within a certain distance increases upward. A ray, M A, Fig. 6, proceeding obliquely downward, will be rendered by refraction more and more nearly horizontal, until it is at length totally reflected, and it is then by successive refractions gradually elevated till it meets the eye of the observer at O, who thus sees an inverted image at M. Fig. 7 shows this effect as seen in the desert, where the eye is cheated by the appearance of water.

When a ray of light enters a piece of glass having parallel sides, as shown in Fig. 8, it is refracted, at the upper surface, downward, as in the case of water, and passes on straight until it reaches the lower surface. But, as it emerges, the reverse effect takes place, and the ray is refracted away from the perpendicular line. Its direction is now parallel to its original course, but it takes the path S instead of S' . The effect of this is, that whenever we look obliquely through plates of glass, as window-panes, all objects seen are slightly displaced, the degree of displacement varying of course with the thickness of the plate.

If, now, we take a piece of glass, of a wedge-shape, in which the sides are not parallel, a different effect is produced by the passage of the light through it. Such a piece of glass, or any transparent substance bounded by surfaces in this way, is called a prism. In Fig. 9, the ray S is represented as striking the prism at I, and, as it enters the glass, it is refracted toward the thicker part, and emerges at E. As it passes out into the air it is again bent in the same direction toward the base of the prism. The dotted lines, N I and E N'  are drawn perpendicular to the faces of the prism, or at right angles, and serve to show that the path of the ray through the prism also makes equal angles with its surfaces.

The lines at which the faces of a prism meet are called its edge. Those in use are generally triangular, and very frequently equilateral, as shown in Fig. 10. For experiment, when used separately, they are commonly mounted upon stands, as shown in Fig. 11, which has several joints. The uppermost is for rotating the prism about its own