CH. xxxiii. VELOCITY OF LIGHT. 327 



plane, and so the light is polarized, as Malus had found it 

 to be from the surface of the Luxembourg windows. He 

 also showed how in some crystals, as in quartz crystals, the 

 waves are made to act upon each other, so that, instead of 

 moving to and fro, they wind round and round like the wire 

 of a corkscrew. These and many other experiments, as, for 

 example, those upon the beautiful colours caused by polar- 

 ization, were carried much farther by the eminent French 

 chemist, M. Biot (born 1774, died 1862), and by Sir David 

 Brewster (born 1784, died 1868), but they are too long and 

 difficult to be explained here. As I said at the beginning 

 of this chapter, the ' Theory of Light ' requires a special 

 study, and if you have understood something of the move- 

 ment of the supposed ether waves how they can interfere 

 with each other and produce light or darkness, how they 

 produce coloured rings in the soap-bubble, and how their 

 vibrations are altered in passing through a crystal or in re- 

 flection at certain angles you have learnt as much as can 

 be easily grasped of the discoveries of Young and Fresnel. 



Velocity of Light in Air and Water Fizeau 

 Foucault. It is necessary, however, to mention here one 

 strong proof of the wave-theory of light although the detailed 

 experiments are too complicated to understand without long 

 explanation. M. Fizeau and M. Leon Foucault, eminent 

 French physicists, have invented two different methods by 

 which the velocity of light can be accurately measured. M. 

 Fizeau's depends upon the rotation of a toothed wheel behind 

 a slit which cuts off a reflected ray, or allows it to pass, accord- 

 ing to the rate at which the wheel is turned. M. Foucault's 

 depends on the rotation of a small mirror revolving round 

 an axis in its own plane like a coin upon its edge, by means 

 of which the spot of light from a ray changes its position 

 perceptibly in the minute interval of time occupied by the 



