DARKNESS FROM TWO LIGHTS. 261 



effects will be actually exhibited by throwing two equal 

 stones into a pool of water, and it will be seen that there 

 are certain lines of a hyperbolic form where the water is 

 quite smooth, in conseqence of the equal waves obliterating 

 one another, while in other adjacent parts the water is 

 raised to a height corresponding to both the waves united. 



In the tides of the ocean, we have a fine example of the 

 same principle. The two immense waves arising from 

 the action of the sun and moon upon the ocean produce 

 our spring-tides by their combination, or when the ele- 

 vations of each coincide, and our neap-tides, when the 

 elevation of the one wave coincides with the depression 

 of the other. If the sun and moon had exerted exactly 

 the same force upon the ocean, or produced tide waves 

 of the same size, then our neap-tides would have disap- 

 peared altogether, and the spring-tide would have been 

 a wave double of the wave produced by the sun and moon 

 separately. An example of the effect of the equality 

 of the two waves occurs in the port of Batsha, where the 

 two waves arrive by channels of different lengths, and 

 actually obliterate each other. 



Now, as sound is produced by undulations or waves in 

 the air, and as light is supposed to be produced by waves 

 or undulations in an ethereal medium, filling all nature, 

 and occupying the pores of transparent bodies, the suc- 

 cessive production of sound and silence, by two loud 

 sounds, or of light and darkness by two bright lights, 

 may be explained in the very same manner as we have 

 explained the increase and the obliteration of waves 

 formed on the surface of water. If this theory of light 

 be correct, then the breadth of a wave of red light will 

 be the 258th thousandth part of an inch, the breadth of 

 a wave of green light the 207th thousandth part of an 

 inch, and the breadth of a wave of violet light the 157th 

 thousandth part of an inch. 



