456 HISTORY OF SCIENCE. 



effect is explained by the undulatory theory of light, he may here again 

 be reminded by the diagram in Fig. 203 of the effect of two systems 

 of interfering waves. We shall suppose the diagram to represent the 

 waves raised on a quiet pool of water by simultaneously dropping in 

 two stones at the points A A. Waves will spread in circles from each 

 point of disturbance as a centre, and the circles in the diagram may 

 be taken as representing the crests of the waves at some one instant of 

 time. The deepest part of the hollows between the waves would of 

 course be half-way between these circles. Now, the two systems of 

 waves would interfere with each other in this way : wherever the crests 

 of two waves coincide, the water would be raised to double the height; 

 and this, it may be observed, occurs at certain series of points, as, for 



204. 



instance, those included in the straight line c c. On the other hand, 

 it will be noticed that in the directions indicated by b b the crest of a 

 wave always occupies the same position as the hollow of a wave be- 

 longing to the other system. The consequence is that at those points 

 the two sets of waves annul each other, and then the water is (for the 

 instant) at its ordinary level. 



In Fig. 204 we have a diagram representing by the lines o M, o N, 

 the edges of two upright metallic plane mirrors, which at o form a very 

 small angle with each other. At L, rays of sunshine, enter the dark 

 room through a very narrow vertical slit, and falling upon the two 

 mirrors, are reflected to a white screen placed at F G. By the laws of 

 ordinary reflection the effects will be precisely the same as if the virtual 

 images of the slit A B were real sources of light. It is a great advan- 

 tage in this arrangement that by merely decreasing the inclination be- 

 tween the mirrors, the two images or light-sources, as we may now 

 consider them, maybe made to approach as near to each other as may 

 be desired. Let us now consider p a point on the screen which re- 

 ceives rays from both sources. It will be obvious that/ is less distant 

 from B than from A, and that the difference will be less as/ is nearer 



