THE SENSATION OF SIGHT. 209 



particles which form the waves of ether, are both enormously 

 greater than that of the waves of water or of air. The waves 

 of light sent forth from the sun differ exceedingly in size, just as 

 the little ripples whose summits are a few inches distant from 

 each other differ from the waves of the ocean, between whose 

 foaming crests lie valleys of sixty or a hundred feet. But, just 

 as high and low, short and long waves, on the surface of water, 

 do not differ in kind, but only in size, so the various waves 

 of light which stream from the sun differ in their height and 

 length, but move all in the same manner, and show (with certain 

 differences depending upon the length of the waves) the same 

 remarkable properties of reflection, refraction, interference, 

 diffraction, and polarisation. Hence we conclude that the 

 undulating movement of the ether is in all of them the same- 

 We must particularly note that the phenomena of interference, 

 under which light is now strengthened, and now obscured by 

 light of the same kind, according to the distance it has traversed, 

 prove that all the rays of light depend upon oscillations of waves ; 

 and further, that the phenomena of polarisation, which differ 

 according to different lateral directions of the rays, show that 

 the particles of ether vibrate at right angles to the direction in 

 which the ray is propagated. 



All the different sorts of rays which T have mentioned 

 produce one effect in common. They raise the temperature of 

 the objects on which they fall, and accordingly are all felt by our 

 skin as rays of heat. 



On the other hand, the eye only perceives one part of these 

 vibrations of ether as light. It is not at all cognisant of the 

 waves of great length, which I have compared with those of the 

 ocean ; these, therefore, are named the dark heat-rays. Such 

 are those which proceed from a warm but not red-hot stove, and 

 which we recognise as heat, but not as light. 



Again, the waves of shortest length, which correspond with 

 the very smallest ripples pi'oduced by a gentle breeze, are so 

 slightly appreciated by the eye, that such rays are also generally 

 regarded as invisible, and are known as the dark cheimcK'vays. 



Between the very long and the very short waves of ether 

 I. p 



