THE PATH OF EVOLUTION 



than when it is of high intensity. The temperature 

 of the atmosphere is thus regulated ; for if the earth 

 radiated its heat away as readily as the solids of the 

 earth receive it from the sun, the temperature even of 

 the tropics would be 200 degrees C. below freezing.* 



The undulations of the ether are excessively rapid. 

 Those which affect the eye and produce vision vary 

 from 370 million million per second in the ultra red 

 rays of the spectrum to 833 million million per 

 second in the ultra violet rays ; these rays have re- 

 spectively a length inversely to the rapidity of vibra- 

 tion of from .0000205 of an inch to .00000914 of an 

 inch, or, in round numbers, say 20 one-millionths 

 of an inch for the extreme red, and a little over 9 

 one-millionths of an inch for the extreme violet. 



These rates of vibration may be better compre- 

 hended when compared with the undulations of the 

 atmosphere, which undulations constitute musical or 

 other sounds, audible to the human ear. Sound is 

 produced by the air being thrown into alternate ex- 

 pansion and contraction by the vibrations of a string, 

 metallic surface, or by the air itself, if set vibrating 

 in a tube with an open end. It propels itself by alter- 

 nate swellings and contractions, as it were, of concen- 

 tric spheres (not as light does by transverse vibra- 

 tions), and travels at the rate of 1089 feet a second. 



*Laiigley in " Barker's Physics.'* 

 126 



