220 OBJECTIONS REMOVED. [SECT. xxni. 



SECTION XXIII. 



Objections to the Undulatory Theory, from a difference in 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 colours 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 favour of that hypo- 

 thesis ; and even cases which at one time seemed unfavour- 

 able to that doctrine have proved upon investigation to 

 proceed from it alone. Such is the erroneous objection 

 which has been made, in consequence of a difference in the 

 mode of action of light and sound, under the same circum- 

 stances, in one particular instance. 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 entering diverges in all directions, and is heard 

 in every part of the room. These phenomena, however, in- 

 stead of being at variance with the undulatory theory, are 

 direct consequences of it, arising from the very great differ- 

 ence 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 diverging from a 

 luminous point enters the hole, the rays round its edges are 

 oblique, and consequently of different lengths, while those 

 in the centre are direct, and nearly or altogether of the same 

 lengths. So that the small undulations between the centre 



