20 



PRESSURE OF LIGHT 



angles to the direction of propagation, that is, 

 perpendicular to the plane of the paper in fig. 5. 

 If we held a horse-shoe magnet with its plane 

 perpendicular to AB, as in fig. 6, then the magnetic 

 lines in the part of the medium between A and B 

 would have the same direction as those between 

 the poles of the magnet when its north pole was 

 in front. The lines between B and C would have 

 the same direction as those of the magnet with 

 its south pole in front, and so on. 



FIG. 6. 



Thus we have in these electro-magnetic waves 

 two sets of tubes, electric and magnetic respect- 

 ively, at right angles to each other and to the line 

 of propagation. Both kinds of tube tend to bulge 

 as it were sideways, tend to press against each 

 other, and tend to press against any surface from 

 which they issue, and against any surface on which 

 they fall. 



Clerk Maxwell showed that on his theory the 

 pressure thus exerted against a square centimetre 



