SECT, xxiii.] OBJECTIONS EEMOVED. 221 



and the edges are in different phases, that is, in different 

 states of undulation. 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 contrary, 

 being very large compared with the hole, do not sensibly 

 diverge in passing through it, and are therefore all so nearly 

 of the same length, and consequently in the same phase, or 

 state of undulation, that none of them interfere sufficiently 

 to destroy one another. Hence all the particles of air in the 

 room are set into a state of vibration, so that the intensity of 

 the sound is very nearly everywhere the same. Strong as the 

 preceding cases may be, the following experiment made by 

 M. Arago, about twenty years ago, seems to be decisive in 

 favour of the undulatory doctrine. Suppose a plano-convex 

 lens of very great radius to be placed upon a plate of very 

 highly polished metal. When a ray of polarized light falls 

 upon this apparatus at a very great angle of incidence, 

 Newton's rings are seen at the point of contact. But, as the 

 polarizing angle of glass differs from that of metal, when the 

 light falls on the lens at the polarizing angle of glass, the 

 black spot and the system of rings vanish. For, although 

 light in abundance continues to be reflected from the surface 

 of the metal, not a ray is reflected from the surface of the 

 glass that is in contact with it, consequently no interference 

 can take place ; which proves, beyond a doubt, that Newton's 

 rings result from the interference of the light reflected from 

 both the surfaces apparently in contact (K 194). 



Notwithstanding the successful adaptation of the un- 

 dulatory system to phenomena, the dispersion of light for a 

 long time offered a formidable objection to that theory, 

 which has only been removed during the present year by 

 Professor Powell of Oxford. 



A sunbeam falling on a prism, instead of being refracted 



