266 THE KETURN. 



The effect is greater when the sun is low, and continual- 

 ly diminishes with its increasing altitude; but the navi- 

 gators' books contain a table in which they can find the 

 proper correction to be made in every case. 



John was quite pleased to find that he could understand 

 this explanation, and the drawing which Lawrence made 

 to represent it, so easily, and said, after a moment's pause, 

 that he thought that he had learned a good deal about 

 light since he had been on that tour. 



" Yes," said Lawrence, " you have indeed. You have 

 made an excellent beginning. But the field of knowledge 

 widens more and more the farther we advance into it. 

 You have learned a great many of the first principles, and 

 these, being fundamental, are of great importance. But 

 when you go farther, and study the construction and phi- 

 losophy of the microscope, the telescope, the magic lan- 

 tern, the stereoscope, and the analysis by the spectroscope 

 of the chemical composition of incandescent substances, 

 however remote from us, you will think that what you 

 have yet learned is, after all, very little. And when you 

 come to investigate the phenomena of diffraction, and in- 

 terference, and polarization, you will almost conclude that 

 you know now nothing at all." 



"What are all those things, any how ?" asked John. 



" What they call diffraction" said Lawrence, " is a 

 change produced in some mysterious -way in the move- 

 ment of rays of light when a very slender beam passes 

 through a very narrow slit or opening, or by the side of a 

 very narrow obstruction, so as to produce fringes of differ- 

 ent colors. You can see these fringes sometimes, though 

 very irregularly, when you look at a bright light with your 

 eyes almost shut, so as to see it through your eyelashes. 

 There are ways of producing them very regularly and 

 beautifully on a screen by means of suitable apparatus, 



