NEW CONCEPTIONS IN SCIENCE 



sand, and called a micro-micron (written with two 

 Greek /z's, /z/z). The shortest light- waves, then, are 

 0.4 \i, or 400 /u/u. The chromatic threads are about 

 % fji t or 250 /u/u, thick. 



This is smaller than any piece of wire can be 

 drawn. With infinite pains Wollaston, a hundred 

 years ago, drew out a platinum wire to 750 /u/u. 

 It still remains the most delicate wire ever drawn. 

 But some of the metals may be hammered out into 

 leaves thinner yet. Gold-leaf is a classic instance. 

 A single grain of gold has been pounded out so 

 as to have a surface of seventy -five square inches. 

 This, it is easy to calculate, makes it not more than 

 8T7?inny f an i ncn through, or about ^ /u, or 66 ^u. 

 It would take about a thousand such sheets of 

 gold, each pressed down solidly on the other, to 

 equal the thickness of this sheet of paper. 



By treating such sheets chemically, Faraday was 

 able to produce gold-leaf that, he says, would re- 

 quire fifty or one hundred thicknesses to equal 

 one wave-length of light. That would have been 

 only four to eight micro -microns (4 /up to 8 ji/i.) 

 Faraday's way of measuring was rather vague, and 

 I do not know if his estimate has ever been verified. 

 He did not care much for mathematics, and the be- 

 wildering equations of mathematical physics puz- 

 zled him as sorely as they do common folk. 



At any rate, Faraday's sheets of gold were so 

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