38 



THE POPULAR SCIENCE MONTHLY, 



density within a certain distance increases upward, A ray, M A^ 

 Fio-. 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 0, 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 dis- 

 placed, the degree of displacement varying of course with the thick- 

 ness of the plate. 



Fig. 8. 



Fio. 9. 



Vision through Glass Plate. 



Eepraction through Pbism. 



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 sub- 

 stance bounded by surfaces in this way, is called a prism. In Fig. 9, 

 the ray S is represented as striking the prism at Z, and, as it enters 

 the glass, it is refracted toward the thicker part, and emerges at JE, 

 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 Zand JE 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 sev- 

 eral joints. The uppermost is for rotating the prism about its own 



