200 OBJECTIONS REMOVED. SECT. XXIII. 



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, seems to be deci- 

 sive 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 th'e surface of the glass that 

 is in contact with it, consoquently 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 sur- 

 faces apparently in contact (N. 199). 



Notwithstanding the successful adaptation of the undulatory 

 system to phenomena, the dispersion of light for a long time 

 offered a formidable objection to that theory, which has been 

 removed by Professor Powell of Oxford. 



A sunbeam falling on a prism, instead of being refracted to a 

 single point of white light, is separated into its component 

 colours, which are dispersed or scattered unequally over a con- 

 siderable space, of which the portion occupied by the red rays is 

 the least, and that over which the violet rays are dispersed is the 

 greatest. Thus the rays of the coloured spectrum, whose waves 

 are of different lengths, have different degrees of refrarigibility, 

 and consequently move with different velocities, either in the 

 medium which conveys the light from the sun, or in the refract- 

 ing medium, or in both ; whereas rays of all colours come from 

 the sun to the earth with the same velocity. If, indeed, 

 the velocities of the various rays were different in space, the 

 aberration of the fixed stars, which is inversely as the velocity, 



