260 LETTERS ON NATURAL MAGIC. 



that the shadow consists of bright and dark stripes sue 

 ceeding each other alternately, the stripe in the very 

 middle or axis of the shadow being a bright one. The 

 rays of light which are bent into the shadow, and which 

 meet in the very middle of the shadow, have exactly the 

 same length of path, so that they form a bright fringe of 

 double the intensity of either ; but the rays which fall 

 upon a point of the shadow at a certain distance from the 

 middle have a difference in the length of their paths, 

 corresponding to the difference at which the lights destroy 

 each other, so that a black stripe is produced on each side 

 of the middle bright one. At a greater distance from the 

 middle, the difference becomes such as to produce a bright 

 stripe, and so on, a bright and a dark stripe succeeding 

 each other to the margin of the shadow. 



The explanation which philosophers have given of these 

 strange phenomena is very satisfactory, and may be easily 

 understood. When a wave is made on the surface of a 

 still pool of water, by plunging a stone into it, the wave 

 advances along the surface, while the water itself is never 

 carried forward, but merely rises into a height and falls 

 into a hollow, each portion of the surface experiencing an 

 elevation and a depression in its turn. If we suppose two 

 waves equal and similar to be produced by two sepa- 

 rate stones, and if they reach the same spot at the same 

 time, that is, if the two elevations should exactly coincide, 

 they would unite their effects and produce a wave twice 

 the size of either ; but if the one wave should be just so 

 far before the other that the hollow of the one coincided 

 with the elevation of the other, and the elevation of the 

 one with the hollow of the other, the two waves would 

 obliterate or destroy one another, the elevation as it were 

 of the one filling up half the hollow of the other, and the 

 hollow of the one taking away half the elevation of the 

 other, so as to reduce the surface to a level. These 



