270 THE RETURN. 



better still, children observe and admire them in the soap 

 bubbles which they blow. The colors come out when the 

 bubble grows so large that the water inclosing it becomes 

 extremely thin. 



"I have seen the colors in the bubbles very often," said 

 John, " but I don't understand how they can be produced 

 by any kind of interference of waves." 



" No," replied Lawrence, " I do not wonder that you do 

 not. It requires a very profound mathematical study to 

 understand it. Newton studied it in that way " 



" What ! with a soap-bubble ?" asked John. 



" Yes," replied Lawrence ; " but the colors moved about 

 so much when the bubble was floating in the open air, and 

 the water dried from the surface so as to cause it to burst 

 so soon, that at first he met with a good deal of difficulty. 

 He saw that it was necessary to contrive some way to 

 remedy these evils, so he blew his bubble in a glass globe, 

 with very transparent sides, which served to protect it 

 from the air, and which he previously filled with moist air 

 in order to prevent the evaporation." 



He found that, when thus covered, the bubble was much 

 more permanent than when exposed in the open air, and 

 the colors arranged themselves in the most symmetrical 

 and beautiful manner. 



" I mean to try it when I get home," said John. 



"I would do so, if I were you," replied Lawrence. 



" Only," said John, " I don't know how I can get a glass 

 globe." 



" Any kind of bottle or jar would do, I suppose," said 

 Lawrence ; " only you must have a stopper, and pass the 

 tube that you blow your bubble with through it, so as to 

 keep the moisture all in the jar, in order to prevent the 

 water of the bubble from evaporating. You must also 

 stop the opening into the pipe, for there is a certain con- 



