THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOUKNAL OF SCIENCE. 



[FIFTH SEEIES.] 



JUNE 1884. 



XL VIII. On the Propagation of an Arbitrary Electro-magnetic 

 Disturbance, on Spherical Waves of Light, and the Dynamical 

 Theory of Diffraction. By Professor Rowland *. 



Intkoduction, 



IN the year 1849 the great paper of Stokes u On the 

 Dynamical Theory of Diffraction " was read before the 

 Cambridge Philosophical Society, and this has remained until 

 the present day the standard upon this important subject. 



The method of Stokes was based upon the old elastic-solid 

 theory of light; and gave the following conclusions : — 



First, That when the incident light was plane-polarized, 

 the diffracted light from a small orifice w r as also plane-polarized 

 in such a manner that the displacement was in the same plane 

 as that of the medium at the orifice. So that if a sphere was 

 drawn with the orifice as a centre, and meridians drawn on 

 the sphere, with the axis in the direction of the vibration at 

 the orifice, then these meridians represented the direction of 

 displacement in the diffracted light. 



Second. The intensity of the polarized light was repre- 

 sented as follows : — Let S represent the angle between the 

 incident ray prolonged and the diffracted ray, and let </> be 

 the angle between the diffracted ray and the direction of the 

 displacement at the orifice. Then the intensity of the dif- 



* From advance sheets of the { American Journal of Mathematics ' 

 communicated bv the Author. 



Phil. Mag. S. 5. Vol. 17. No. 108. June 1884. 2 F 



