SECT. XX1JI. OBJECTIONS REMOVED. 191 



contrary, being very large compared with the hole, da 

 not sensibly diverge hi passing through it. and are there- 

 fore 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 pre- 

 ceding cases may be, the following experiment made by 

 M. Arago about twenty years ago seems to be decisive 

 in favor 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 sys- 

 tem 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 New- 

 ton's rings result from the interference of the light re- 

 flected from both the surfaces apparently in contact (N. 

 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 re- 

 fracted to a single point of white light, is separated into 

 its component colors, which are dispersed or scattered 

 unequally over a considerable 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 colored spectrum whose waves are of differ- 

 ent lengths, have different degrees of refrangibility, and 

 consequently move with different velocities, either in the 

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

 refracting medium, or in both ; whereas rays of all colors 



