140 COLOURS OF THIN PLATES. 



being in complete accordance when the interval of retarda- 

 tion is an even multiple of the length of halt a wave, and in 

 complete discordance when that interval is an odd multiple of 

 the same quantity, it would seem, from the foregoing account, 

 that the bright rings should be formed at all those points for 

 which n is an even number in the formula above given (or 

 the thickness an even multiple of jX sec 0,) and the dark 

 rings at those points for which it is odd. If this were true, 

 the point of contact should be a point of accordance, and the 

 rings should commence from a bright centre, instead of a dark 

 one. 



This apparent discrepancy is explained by the fact, that 

 the two reflexions take place under opposite circumstances, 

 one of the rays being reflected at the surface of a denser, and 

 the other at that of a rarer medium. 



The effect of this difference will be best understood by a 

 simple illustration. When one elastic ball strikes another at 

 rest, it communicates motion to it in all cases ; but its own 

 condition after the shock will depend on the relative masses 

 of the two balls. If the balls be equal, the first will remain 

 at rest after the shock. If they be unequal, it will move ; 

 and its motion will be in the direction of its former motion, 

 when its mass exceeds that of the second ball, it will be in 

 the opposite direction when it is less. This will help us to 

 understand what passes when a wave reaches the surface 

 separating two media. The particles of ether next the 

 bounding surface communicate motion to the adjacent par- 

 ticles of the second medium, and thus give rise to the re- 

 fracted wave. But the former particles will not remain at 

 rest afterwards, unless the density and elasticity of the ether 

 be the same in the two media. When this is not the case, 

 the particles of the first medium will move, after communi- 

 cating motion to those of the second, and, in moving, give 

 rise to the reflected wave. Thus refraction is always accom- 



