18 



PRESSURE OF LIGHT 



pose that there is a spinning motion of the con- 

 stituents of the medium round the line of force 

 passing through them, perhaps a spinning of the 

 atoms, more probably a spinning of the corpuscles 

 round the atoms. 



A spinning body like the Earth tends to draw 

 together along the axis of spin, and to press out 

 sideways at the Equator. Similarly the spinning 

 medium round a magnetic line of force tends to 



FIG. 5. 



draw together lengthways and to bulge out sideways, 

 and so we have side pressure of the tubes of mag- 

 netic force against each other just as we have 

 it with tubes of electric force, though probably it 

 is now produced in a different way. 



Now let us see what electric and magnetic con- 

 ditions we must imagine in a train of light waves. 



Imagine that we could travel on with the waves 

 and at their speed, so that we could always have 

 the same waves under observation, and let us sym- 



