CHAPTER II. 



THE MECHANICAL PRESSURE OF LIGHT AND ITS 

 CONSEQUENCES. 



To the reader who has not followed, particularly, the 

 course of scientific advance during the last ten years, the 

 heading of this chapter must seem upsetting to all accepted 

 notions as to the nature of light. By light is meant un- 

 dulations in the omnipresent ether and it seems odd to 

 attribute mechanical pressure to such an immaterial thing. 

 Yet in 1873 Clerk-Maxwell, in one of his prescient mathe- 

 matical inspirations showed that such a pressure should 

 exist, and his conclusion was mathematically borne out in 

 1876 by Bartoli on totally different grounds. Briefly it 

 may be stated that this light pressure p is determined if 

 we know the amount of energy E in the light, the reflect- 

 ing power of the substance that receives it r, and v the 

 velocity of light, for then 



P-f(lXr) 



The experimental verification, as is often the case, lagged 

 long behind the mathematical prediction. In 1901, how- 

 ever, Peter Lebedew actually proved and measured the 

 mechanical pressure of light. The pressure discovered was 

 small, but the smallness of a thing is often an inverse 

 measure of its importance, and as this light pressure has 

 been found adequate to the task of explaining some of 

 earth's greatest mysteries we shall indicate his method 

 here. Briefly, he allowed a beam of light to fall on a sus- 

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