258 LETTERS ON NATURAL MAGIC. 



vibrations in a second, they will produce each equal 

 waves of sound, and these waves will conspire in gene- 

 rating an uninterrupted sound, double of either of the 

 sounds heard separately. If the two strings or the two 

 columns of air are not in unison, but nearly so, as in the 

 case where the one vibrates 100, and the other 101 times 

 in a second, then at the first vibration the two sounds 

 will form one of double the strength of either ; but the 

 one will gradually gain upon the other, till at the fiftieth 

 vibration it has gained half a vibration on the other. At 

 this instant the two sounds will destroy one another , and an 

 interval of perfect silence will take place. The sound 

 will instantly commence, and gradually increase till it 

 becomes loudest at the hundredth vibration, where the 

 two vibrations conspire in producing a sound double of 

 either. An interval of silence will again occur at the 

 150th, 250th, 350th vibration, or every second, while a 

 sound of double the strength of either will be heard at 

 the 200dth, SOOdth, and 400dth vibration. When the 

 unison is very defective, or when there is a great dif- 

 ference between the number of vibrations which the two 

 strings or columns of air perform in a second, the suc- 

 cessive sounds and intervals of silence resemble a rattle. 

 With a powerful organ the effect of this experiment is very 

 fine, the repetition of the sounds wow wow wow repre- 

 senting the doubled sound and the interval the silence 

 arising from the total extinction of the two separate sounds. 

 The phenomena corresponding to this in the case of 

 light are perhaps still more surprising. If a beam of red 

 light issues from a luminous point, and falls upon the 

 retina, we shall see distinctly the luminous object from 

 which it proceeds ; but if another pencil of red light 

 issues from another luminous point anyhow situated, 

 provided the difference between its distance and that of 

 the other luminous point from the point of the retina, on 



