Chap, xxiv.] THE VELOCITY OF LIGHT. 299 



Specially as regards light, the intensity depends 

 on the amplitude of the vibrations of the luminous 

 body. The frequency of vibrations will be found to 

 determine the difference in colours, red being pro- 

 duced by vibrations of less frequency, or, what is the 

 same thing, by longer wave lengths, than the vibra- 

 tions producing yellow or violet. This is referred to 

 again in speaking of colour. (See chap, xxvi.) 



Self-luminous bodies, then, are bodies in the 

 state 01 vibration to produce light. TRANSPARENT 

 bodies are those which transmit the vibrations so that 

 on reaching the *eye they produce images of the 

 object ; while TRANSLUCENT bodies permit the passage 

 of the vibrations, but so that the body from which 

 they proceed cannot be distinguished. OPAQUE bodies 

 do not transmit the vibrations, but reflect them. 



Light is propagated in straight lines. It is thus 

 that an opaque body casts a shadow, since it intercepts 

 the light and causes the space immediately behind it 

 to be devoid of light. ? 



The velocity of light has been calculated by 

 various experimenters. Fizeau's method consists in 

 placing a plane mirror directly in front of a source of 

 light, but] at a great distance from it. An observer, 



o ' e O ' 



stationed behind the light, perceives the beam reflected 

 from the mirror, that is, after it has travelled from 

 the light to the mirror and back again. In front of 

 the source of light is a toothed wheel capable of 

 being revolved with a varying degree of rapidity. 

 The wheel may be turned at such a rate that a beam 

 of light travelling from the source may pass in the 

 space between two teeth and be reflected in time to 

 be intercepted by a tooth, so that the light will be 

 invisible. Thus, from the rapidity of the wheel's 

 revolution, and the number of teeth, the time occupied 

 by the light in travelling to the mirror and back again 

 can be estimated, and, the distance being known, the 



