82 INTERMINGLING OF UNDULATIONS. 



cept so far as they are intercepted in their passage and re* 

 fleeted, so as to come to us and enter our eyes, where they 

 form an image on the retina and produce vision." 



When Lawrence had said this, he took out his watch, 

 looked at it, and said, 



"Well, we have been here looking at the ducks, and talk- 

 ing about the intermingling of vibrations and undulations, 

 about twenty minutes, or twenty-five. Shall we call this 

 time study hours or not ?" 



"That is just as you say," replied John. "I've learned 

 something, at any rate, and something that I did not know 

 before." 



"And you have listened attentively to it," replied Law- 

 rence. "I think it would be fair to consider it study hours. 

 And there is one thing more that I should like to explain, 

 which will take about five minutes, if you are not too tired, 

 and that will just make up the half hour." 



John said he was not too tired, and would like very well 

 to make up the half hour. So Lawrence explained that, 

 although the emanations of light, whether they were really 

 of the nature of undulations or not, did not appear, in ordi- 

 nary cases, to interfere with each other at all, however nu- 

 merous and complicated their intercrossings might be, that 

 still there was an optical phenomenon which was called in- 

 terference, although, strictly speaking, it was not interfer- 

 ence in the common sense, since both of the radiations in 

 these cases produced its own full effect, though the two 

 effects combined produced a somewhat remarkable result. 



Lawrence explained the principle by asking John to im- 

 agine that two ducks were swimming over the water in 

 such directions that the undulations should not cross each 

 other, but should follow each other in long lines exactly 

 parallel. 



"Now we may suppose," he added, "that these lines are 



