from a denser to a rarer medium, and consists in a bending of the incident ray toward the normal to the surface of the denser medium. Suppose we have a plate of glass, for example, and a ray of light falling upon the surface in any direction. According to the corpuscular theory, the substanceFIG. 41 below the surface exerts an attraction upon the light corpuscles. Such attraction can act only in the direction of the normal. If we separate it into two components, one in the surface and one normal to it, the normal one will be increased. These two components might be represented by OA and OB in Fig. 41, and the resultant of the two would be OC. In consequence of the presence of the denser medium, the normal component of the velocity of the particle is increased, and the resultant is now OC', which is greater than OC.

Let us next consider refraction according to the wave theory. A wave front ab (Fig. 42) is approaching the surface ac of a denser medium in the direction bc. This direction is changed by refraction to ce, and the corresponding direction of the new wave front is cd. During the time that the wave ab moves through the distance bc in the rarer medium, it moves through the smaller distance ad in the denser. Thus the results, according to the two theories, are exactly reversed.

Hence, if we could measure the enormous speed of light—about 400,000 times as great as that of a rifle bullet—it would be possible to put the two theories to the test. In order to