346 THE PRINCIPLES OF SCIENCE. 



once obtained experimental data, there is no further 

 difficulty bevond that of arithmetic or algebraic calcu- 



V / 



lation. 



Gold is reduced by the gold-beater to leaves so thin, 

 that the most powerful microscope would not detect any 

 measurable thickness. If we laid several hundred leaves 

 upon each other to multiply the thickness, we should 

 still have no more than Y^th of an inch at the most to 

 measure, and the errors arising in the superposition and 

 measurement would be considerable. But we can readily 

 obtain an exact result through the connected amount of 

 weight. Faraday weighed 2000 leaves of gold, each 

 3f inch square, and found them equal to 384 grains. 

 From the known specific gravity of gold, it was easy to 

 calculate that the average thickness of the leaves was 

 2 8 2*0 oo of an inch g. 



We must ascribe to Newton the honour of leading the 

 way in methods of minute measurement. He did not 

 call waves of light by their right name, and did not 

 understand their nature ; yet he measured their length, 

 though it did not exceed the 2,ooo,oooth part of a metre 

 or the one fifty thousandth part of an inch. He pressed 

 together two lenses of very large but known radii. It 

 was not difficult to calculate the interval between the 

 lenses at any point, by measuring the distance from the 

 central point of contact. Now, with homogeneous rays the 

 successive rings of light and darkness mark the points at 

 which the interval between the lenses is equal to one 

 half, or any multiple of half a vibration of the light, so 

 that the length of the vibration became known. In a 

 similar manner many phenomena of interference of rays 

 of light admit of the measurement of the wave lengths. 

 The fringes of interference arise from rays of light which 

 cross each other at a small angle, and an excessively 

 K Faraday, 'Chemical Researches,' p. 393. 



