OUR PHYSICAL WORLD 



to travel from i to 2, so that the latter has not completed its 

 revolution but is at the point indicated in its orbit. The posi- 

 tions of 3, 4, 5, etc., are also indicated, and are connected by the 

 solid line i to 9 that outlines the wave form from crest to crest. 

 The height of the wave is the long diameter of a molecular orbit. 

 The wave form advances from left to right. 



When a light wave enters a glass prism as in Figure 151 the 

 bottom of the wave encounters the glass and is retarded while 

 the top continues to move at its initial velocity somewhat as 





FIG. 154. Diagram of marching men to illustrate refraction and dispersion of 



light. 



happens in the case of a water-wave when it strikes a shelving 

 shore. The direction of advance is therefore altered, or, as we 

 say, the light is refracted. On leaving the prism in our diagram, 

 it is the top of the wave that emerges first and so moves with 

 increased rapidity, since it is now in a less dense medium, while 

 the bottom is still retarded, and so the course of the ray of light 

 is again altered. 



Suppose a line of marching men be shown by circles (see Fig. 

 154). In their path is a wedge-shaped area of deep sand on an 

 otherwise hard surface. As the line strikes the difficult going 



