THE SIZE OF ATOMS. 153 



and there i/ioooth of a millimetre, and there a 

 round atom of oxygen i/i,ooo,oooth of a milli- 

 metre in diameter. You see them all. 



Now we must have a practical means of mea- 

 suring, and optics supply us with it, for thou- 

 sandths of a millimetre. One of our temporary 

 standards of measurement shall be the wave-length 

 of light ; but the wave-length is a very indefinite 

 measurement, because there are different wave- 

 lengths for different colours of light, visible and 

 invisible, in the ratio of I to 16. We have, as it 

 were borrowing an analogy from sound four 

 octaves of light that we know of. How far the 

 range in reality extends, above and below the 

 range hitherto measured, we cannot even guess 

 in the present state of science. The table before 

 you (Table I.) gives you an idea of magnitudes 

 of length, and again of small intervals of time. 

 In the column on the left you have the wave- 

 length of light in fractions of a centimetre ; the 

 unit in which these numbers is measured being 

 the i/ioo,oooth (or io~ 5 ) of a centimetre. We 

 have then, of visible light, wave-lengths from 



