HOW LIGHT EXERTS PRESSURE IQ 



bolize them in the usual conventional way by the 

 curve ABODE in fig. 5. 



The actual electric condition between A and B 

 will be the same as that between a positively 

 electrified plate P and a negatively electrified plate 

 Q, while that between B and C will be the same as 

 that between a positively electrified plate R and a 

 negatively electrified plate S. CD will be like AB, 

 and DE like BC. 



Of course there is nothing corresponding to the 

 electrified plates in the actual waves, but the 

 medium in their course is modified just as the 

 medium between the successive pairs of plates is 

 modified. The length of a wave includes two 

 oppositely affected halves, i.e. stretches from A to 

 C, and for ordinary light a wave is somewhere 

 about a fifty-thousandth of an inch long. 



These waves are travelling on, say from left to 

 right, and the changed condition of the medium 

 symbolized by the lines of force is being propa- 

 gated on from point to point in their course. 

 But to propagate a condition we must have motion 

 in the machinery which propagates it. 



This motion of the machinery is supplied by the 

 spinning round the magnetic lines of force, which 

 must accompany the electric lines to make their 

 propagation possible. We know from various 

 experiments that the magnetic lines must be at 

 right angles to the electric lines and also at right 



B 2 



