268 THE KETUKN. 



" It could not be done," said John. 



" True," said Lawrence ; " but can it not be imagined ?" 



" I don't think I can hardly imagine it," said John. 



" Not even as an illustration ?" said Lawrence. 



"I don't know," said John, speaking doubtfully. 



" You'll have to imagine it," said Lawrence, " if you wish 

 to get an idea of what is meant by interference in the case 

 of light. Besides, though you say it would be impossible, 

 perhaps, to do this with waves of water, the effect can be 

 produced exactly by a mechanical apparatus to make arti- 

 ficial representations of waves." 



" I should like to see that apparatus," said John. 



"At any rate," continued Lawrence, "it is found that 

 rays of light, or luminous impulses following each other in 

 a certain way, do extinguish each other. The experiments 

 are very complicated and very curious, but they are thought 

 to prove positively that light really consists of a rapid suc- 

 cession of some kind of undulations or waves." 



" Do you think they do really prove that ?" asked John. 



" I think they prove the existence of some kind of inter- 

 mittent action, with alternating conditions capable of in- 

 tensifying or neutralizing each other, according as they 

 agree or disagree ; but whether the successive impulses 

 are of the character of vibrations or undulations in a sub- 

 tle ether, I do not know." 



Lawrence was right, perhaps, in saying that he was not 

 entirely satisfied in respect to the precise nature of this 

 mysterious action ; but, at any rate, it seems to be proved 

 that there is an excessively rapid intermittent force of 

 some kind or other that is concerned in the production of 

 light, and the length of the several pulses, and the number 

 which are produced in a second, seem to have been quite 

 exactly ascertained, on the principle of determining the 

 interval in time and distance which is requisite to produce 



