198 LETTEES ON NATURAL MAGIC. 



receiving the light on a sheet of paper. If we 

 hold a needle or piece of slender wire in this 

 light, and examine its shadow, we shall find that 

 the shadow consists of bright and dark stripes 

 succeeding 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 differ- 

 ence in the length of their paths, corresponding 

 to the difference at which the lights destroy each 

 other, so that a Uack 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 ex- 

 periencing an elevation and a depression in its 

 turn. If we suppose two waves equal and similar 

 to be produced by two separate 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 



