THE SEXSATIOX OF SIGHT. 237 



the little ripples whose summits are a few inches distant 

 from each other differ from the waves of the ocean, be- 

 tween whose foaming crests lie valleys of sixty or a hun- 

 dred 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 depend- 

 ing upon the length of the waves) the same remark- 

 able properties of reflection, refraction, interference, dif- 

 fraction, 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 par- 

 ticles of ether vibrate at right angles to the direction in 

 which the ray is propagated. 



All the different sorts of rays which I have mentioned 

 produce one effect in common. They raise the tempera- 

 ture 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 cogni- 

 sant 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 produced by a gentle breeze, 

 are so slightly appreciated by the eye, that such rays are 



