3<X> ON LIGHT. 



of one and the same "channel" (using these terms also 

 in their most general sense) along which each set, separ- 

 ately, might be freely propagated i.e., so that the fore- 

 most crest of the first set shall strike the mouth of the 

 channel at the same moment with that of the other they 

 will combine and run on along the channel as a single 

 set or series of waves of double the height or intensity. 

 In this case they are said to arrive "in the same phase" 

 (a term borrowed from the phases of the moon which 

 passes periodically through the states of full and new, 

 increase and wane). The same will be the case if the 

 foremost crest of the series be so timed (by the previous 

 arrangements) as to reach the mouth of the channel 

 simultaneously with the second, third, or fourth crest of 

 the other, in which case the one set is said to be in 

 arrear or advance of the other (as the case may be) by 

 one, two, or more entire " undulations" On the other 

 hand, if the foremost crest of the one set be so timed as to 

 arrive simultaneously with the first, second, third, &c., 

 trough of the other up to the time of its arrival indeed 

 the one, two, or three foremost waves which are not con- 

 tradicted will run forward ; but, from the moment when 

 the others begin to arrive, they will cease to be followed 

 up by any more. In this case the one set is in advance 

 or arrear of the other by exactly one, or three, or five, 

 &c., semi-undulations , and the series are said to be in 

 opposite phases. In the intermediate " phases " it is easy 

 to see that a combined set of waves will be pro- 

 duced, but intermediate in height, intensity, or (as 

 it is called) "amplitude" between these two extremes 



