THE WA VE THEORY OF LIGHT. 327 



air. Comets make a ; disturbance in the air, and 

 perhaps the luminiferous ether is split up by the 

 motion of a comet through it. So when we ex- 

 plain the nature of electricity, we explain it by 

 a motion of the luminiferous ether. We cannot 

 say that it is electricity. What can this lumini- 

 ferous ether be ? It is' something that the planets 

 move through with the greatest ease. It permeates 

 our air ; it is nearly in the same condition, so far 

 as our means of judging are concerned, in our air 

 and in the inter-planetary space. The air disturbs 

 it but little ; you may reduce air by air-pumps to 

 the hundred thousandth of its density, and you 

 make little effect in the transmission of light 

 through it. The luminiferous ether is an elastic 

 solid, for which the nearest analogy I can give you 

 is this jelly which you see, 1 and the nearest analogy 

 to the waves of light is the motion, which you 

 can imagine, of this elastic jelly, with a ball of 

 wood floating in the middle of it. Look there, 

 when with my hand I vibrate the little red ball 



1 Exhibiting a large bowl of clear jelly with a small red wooden 

 ball embedded in the surface near the centre. 



