278 The Outline of Science 



of light was underrated We now know the distance, and we 

 easily get the velocity of light. 



Xo doubt it seems far more wonderful to discover this within 

 the walls of a laboratory, but it was done as long ago as 1850. 

 A cogged wheel is so mounted that a ray of light passes between 

 two of the teeth and is reflected back from a mirror. Now, slight 

 as is the fraction of a second which light takes to travel that dis- 

 tance, it is possible to give such speed to the wheel that the next 

 tooth catches the ray of light on its return and cuts it off. The 

 speed is increased still further until the ray of light returns to the 

 eye of the observer through the notch next to the one by which 

 it had passed to the mirror! The speed of the wheel was known, 

 and it was thus possible again to gather the velocity of light. If 

 the shortest waves are ^T.W of an inch in length, and light 

 travels at 186,000 miles a second, any person can work out that 

 about 800 trillion waves enter the eye in a second when we see 

 "violet." 



Sorting out Light-waves 



The waves sent out on every side by the energetic electrons 

 become faintly visible to us when they reach about ^^/ornr of an 

 inch. As they become shorter and more rapid, as the electrons 

 increase their speed, we get, in succession, the colours red, orange, 

 yellow, green, blue, indigo, and violet. Each distinct sensation 

 of colour means a wave of different length. When they are all 

 mingled together, as in the light of the sun, we get white light. 

 When this white light passes through glass, the speed of the 

 waves is lessened; and, if the ray of light falls obliquely on a 

 triangular piece of glass, the waves of different lengths part com- 

 pany as they travel through it, and the light is spread out in a 

 band of rainbow-colour. The waves are sorted out according to 

 their lengths in the "obstacle race" through the glass. Anyone 

 may see this for himself by holding up a wedge-shaped piece of 

 crystal between the sunlight and the eye; the prism separates the 



