190 OBJECTIONS REMOVED. SEC*. XXIII. 



SECTION XXIII. 



Objections to the Undulatory Theory, from a Difference iu the Action of 

 Sound and Light under the same circumstances, removed The Disper- 

 sion of Light according to the Undulatory Theory. 



THE numerous phenomena of periodical colors arising 

 from the interference of light, which do not admit of 

 satisfactory explanation on any other principle than the 

 undulatory theory, are the strongest arguments in favor 

 of that hypothesis ; and even cases which at one time 

 seemed unfavorable to that doctrine have proved upon 

 investigation to proceed from it alone. Such is the er- 

 roneous objection which has been made, in consequence 

 of a difference in the mode of action of light and sound, 

 under the same circumstances, in one particular in- 

 stance. When a ray of light from a luminous point, 

 and a diverging sound, are both transmitted through a 

 very small hole into a dark room, the light goes straight 

 forward and illuminates a small spot on the opposite wall, 

 leaving the rest in darkness ; whereas the sound on en- 

 tering diverges in all directions, and is heard in every 

 part of the room. These phenomena, however, instead 

 of being at variance with the undulatory theoiy, are 

 direct consequences of it, arising from the very great 

 difference between the magnitude of the undulations of 

 sound and those of light. The undulations of light are 

 incomparably less than the minute aperture, while those 

 of sound are much greater. Therefore when light di- 

 verging from a luminous point enters the hole, the rays 

 round its edges are oblique, and consequently of different 

 lengths, while those in the center are direct, and nearly 

 or altogether of the same lengths. So that the small 

 undulations between the center and the edges are in 

 different phases, that is, in different states of undula- 

 tion. Therefore the greater number of them interfere, 

 and by destroying one another produce darkness all 

 around the edges of the aperture ; whereas the central 

 rays having the same phases, combine, and produce a 

 spot of bright light on a wall or screen directly opposite 

 the hole. The waves of air producing sound, on the 



