322 NINETEENTH CENTURY. FT. m. 



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will be that of red. But the bubble is always growing 

 gradually thicker down its sides because the soapy liquid is 

 creeping downwards. So a little lower down the red waves 

 from the two surfaces a and b will no longer fit each other, 

 but will meet unevenly and the red colour will be destroyed. 

 It will now be the turn of the violet rays to combine and 

 make a strong wave to our eye ; a little lower down it will 

 be the turn of the green waves, then of the yellow, and then 

 the film will be thick enough for the red waves to come 

 together again, and so it will go on ; each colour in its 

 turn will produce a strong wave, while all the others are 

 quenched, until the film is too thick for the effect to be 

 produced. 



This is a very rough idea of the way in which the Undu- 

 latory Theory explains the colours which we see in shadows 

 and in the soap-bubble. When you study the subject of 

 light you will see how very complicated these wave move- 

 ments really are ; but without special knowledge you cannot 

 understand more than I have given you here. The colours 

 on mother-of-pearl, on a duck's neck, on the transparent 

 wings of insects, and even on the scum floating on a pond, 

 are all produced by the interference of light, and we owe 

 the discovery of this simple and beautiful explanation to 

 Dr. Thomas Young. 



Mai us discovers the Polarization of Light by Re- 

 flection, 1808. The next step in the science of light was 

 made by Etienne Louis Malus, a young French engineer 

 officer, who was born in 1775, and died in 1812, when he 

 was only thirty-seven years of age. He was a most accom- 

 plished mathematician, and if he had lived longer, would 

 probably have been one of the most celebrated men of our 

 century. 



You will remember that in 1 669 a Danish physician named 



