138 IXTERFERENCE OF LIGHT. 



arisen the discovery of one of the most remarkable con- 

 ditions within the range of physical science. 



Two bright lights may be made to produce darkness. 

 If two pencils of light radiate from two spots very 

 close to each other in such a manner that they cross 

 each other at a given point, any object placed at that 

 line of interference will be illnminated with the sum 

 of the two luminous pencils. If we suppose those rays 

 to move in waves, and the elevation of the wave to 

 represent the maximum of luminous effect, then the 

 two waves meeting, when they are both at the height of 

 their undulation, will necessarily produce a spot of 

 greater intensity. If now we so arrange the points of 

 radiation, that the systems of luminous waves pro- 

 ceed irregularly, and that one arrives at the screen 

 half an undiilation before the other, the one in 

 elevation falling into the depression of the other, a 

 mutual annihilation is the consequence. This fact, 

 paradoxical as it may appear, was broadly stated by 

 Grimaldi, in the description of his experiments on the 

 inflection of light, and has been observed by many 

 others. The vibratory hypothesis, seizing upon the 

 analogy presented by two systems of waves in water, 

 explains this plausibly, and many similar phenomena of 

 what is called the interference of light ; but still upon 

 examination it does not appear that the explanation is 

 quite free from objection.* 



Another theory, not altogether new to us, it being in- 

 dicated in Mayer's hypothesis of three primary colours 

 (1775), and to be found as a problem in some of the 

 Encyclopaedias of the last century, has been put forth, 

 in a very original manner, by that master-mind of intel- 

 lectual Germany, Goethe ; and from the very compre- 

 hensive views which this poet-philosopher has taken of 



* Brewster's Optics : Lardner's Cabinet Cyclopaedia. Herschel, 

 Qn Light: Encyclopaedia Metropolitana. 



