INTERFERENCE OF LIGHT. 81 



nishes a striking instance of these effects. The tidal wave 

 reaches this port by two distinct channels, which are so un- 

 equal in length, that the time of arrival by one passage is 

 exactly six hours longer than by the other. It follows from 

 this that when the crest of the tidal wave, or the high water, 

 reaches the port by one channel, it is met by the low water 

 coming through the other ; and when these opposite effects 

 are also equal, they completely neutralize each other. At 

 particular seasons, therefore, when the morning and evening 

 tides are equal, there is no tide whatever in the port of Batsha ; 

 while at other seasons there is but one tide in the day, wiiose 

 height is the difference of the heights of the ordinary morn- 

 ing and evening tides. 



Analogous phenomena take place in sound, and produce 

 the coincidences or beats in music. These effects occur when 

 the condensed part of the aerial pulse, arising from one origin 

 of sound, coincides with the rarified part of that proceeding 

 from the other. They are often heard during the playing of 

 a large organ, and give rise to the swelling and falling sounds 

 which are heard, especially among the lower notes of the 

 instrument. 



(101) That two lights, then, should produce darkness, is a 

 result of the same kind as that two sounds should cause silence, 

 or that two waves should make a dead level. But we are not 

 left to analogy alone for the proof of this remarkable conse- 

 quence of the wave-theory of light. The phenomenon itself 

 has been established by the most direct and convincing ex-, 

 periments ; and we shall soon see that it is observed in a 

 multitude of cases where its existence was at first little sus- 

 pected. 



This important law now known under the name of the 

 interference of light was for the first time distinctly stated 

 and established by Young, although some facts connected 



