Foundations of the Universe 275 



sun to travel that 93,000,000 miles odd which separates it from 

 our earth. Besides the fact that light takes time to travel, it can 

 be shown that light travels in the form of waves. We know that 

 sound travels in waves; sound consists of waves in the air, or 

 water or wood or whatever medium we hear it through. If an 

 electric bell be put in a glass jar and the air be pumped out of 

 the jar, the sound of the bell becomes feebler and feebler until, 

 when enough air has been taken out, we do not hear the bell at 

 all. Sound cannot travel in a vacuum. We continue to see the 

 bell, however, so that evidently light can travel in a vacuum. The 

 invisible medium through which the waves of light travel is the 

 ether, and this ether permeates all space and all matter. Between 

 us and the stars stretch vast regions empty of all matter. But 

 we see the stars; their light reaches us, even though it may take 

 centuries to do so. We conceive, then, that it is the universal 

 ether which conveys that light. All the energy which has reached 

 the earth from the sun and which, stored for ages in our coal- 

 fields, is now used to propel our trains and steamships, to heat 

 and light our cities, to perform all the multifarious tasks of 

 modem life, was conveyed by the ether. Without that universal 

 carrier of energy we should have nothing but a stagnant, lifeless 

 world. 



We have said that light consists of waves. The ether may 

 be considered as resembling, in some respects, a jelly. It can 

 transmit vibrations. The waves of light are really excessively 

 small ripples, measuring from crest to crest. The distance from 

 crest to crest of the ripples in a pond is sometimes no more than 

 an inch or two. This distance is enormously great compared to 

 the longest of the wave-lengths that constitute light. We say 

 the longest, for the waves of light differ in length; the colour de- 

 pends upon the length of the light. Red light has the longest 

 waves and violet the shortest. The longest waves, the waves of 

 deep-red light, are seven two hundred and fifty thousandths of 

 an inch in length ( m ] m inch) . This is nearly twice the length 



