6 TWO NEW WORLDS 



in a lateral direction. A microscope has been 

 constructed on this principle of lateral illumination 

 by Siedentopf and Szigmondy, 1 and by its means 

 objects only a millionth of an inch in diameter 

 have been perceived. We cannot say that these 

 objects have been "seen." But their presence has 

 been indicated by what might be called diffractive 

 symbols, appearances ser^ig to indicate their 

 presence in the field of vision, just as distant 

 smoke indicates a fire. Thus, when in the "ultra- 

 microscope" we perceive an object surrounded by 

 rings, we know that we have to do with a very 

 minute body. We can count such bodies, and 

 observe their motions and changes of size and 

 arrangement. The inventors of the ultra-microscope 

 used their instrument for determining the weight 

 of the particles of gold contained in a colloid 

 solution of that substance. 



The world revealed by the ultra-microscope is 

 not a living world. No organisms can fall short 

 of a certain size and live. Life appears to require 

 a certain minimum of molecules to support it. The 

 physical processes of life are so manifold that a 

 single molecule, or even a million molecules, are 

 unable to compass them. 



No man has yet seen a molecule; but countless 

 experiments and indirect measurements have given 

 us a fair idea of their size and weight, and we 



1 Annalen der Physik, No. 1, 1903. 



