POLARIZATION. INTERFERENCE. 267 



but to understand clearly how they are produced requires 

 a great deal of mathematical knowledge. It is in some 

 way by which the pulsations or vibrations of different rays 

 mingle or combine their actions so as to produce new and 

 strange effects. Sometimes two rays entirely neutralize 

 each other, so that two lights make darkness. This is 

 what they call interference. 



"As to polarization" continued Lawrence, "that is more 

 difficult still to understand. It forms quite a science by it- 

 self, and one, too, of a highly mathematical character. Po- 

 larized light is light which has been changed in a certain 

 way, so that it acts differently from light in its ordinary 

 state, and produces certain beautiful and very wonderful 

 effects in the microscope, and reveals in a marvelous man- 

 ner certain differences in the internal constitution of dif- 

 ferent transparent substances which could be discovered 

 in no other way. 



" What they call interference," continued Lawrence, " is, 

 as I have already said, a kind of combination of the waves, 

 by which the swell of one is made to correspond with the 

 hollow of another, as it were, and so they are both extin- 

 guished. Imagine that you throw a stone into a pond and 

 set in motion circles of waves, and then suppose that an- 

 other stone is thrown in so as to strike at precisely the 

 right instant to make a second set of waves that shall ex- 

 actly coincide with the first set. This would tend to in- 

 crease the height of them." 



"Yes," said John, "I admit that." 



"But now," continued Lawrence, "suppose the stone 

 were thrown in at the right instant to make the hollows 

 of the second set coincide with the swellings of the first. 

 The two sets of impulses would then neutralize each other 

 or interfere, as they call it when speaking of light, and 

 the water would remain level." 



