Dr. Oliver Lodge on Opacity. 411 



That explains the whole paradox — there is the reflected 

 beam to be considered too. At the entering boundary the 

 incident and reflected amplitudes are in opposite phase, and 

 nearly equal, and their algebraic sum, which is transmitted, 

 is small. At the emerging boundary the incident and re- 

 flected amplitudes are in the same phase, and nearly equal, 

 and their algebraic sum, which is transmitted, is large — is 

 nearly double either of them. But it is a curious action : — 

 either more light is pushed out from the limiting boundary 

 of a conductor than reaches it inside, or else, 1 suppose, 

 the Telocity of light inside the metal must be greater 

 than it is outside, a result not contradicted by Kundt's 

 refraction experiments, and suggested by most optical 

 theories. It is worth writing out the slab theory a little 

 more fully, to make sure there is no mistake, though the 

 whole truth of the behaviour of bodies to light can hardly be 

 reached without a comprehensive molecular dispersion theory. 

 I do not think Mr. Heaviside has published his slab theory 

 anywhere yet. A slab theory is worked out by Prof. J. J. 

 Thomson in Proc. Roy. Soc. vol. xlv., but it has partly 

 for its object the discrimination between Maxwell's and other 

 rival theories, so it is not very simple. Lord Kelvin's Balti- 

 more lectures probably contain a treatment of the matter. 

 All that I am doing, or think it necessary to do in an 

 Address, is to put in palatable form matter already to a few 

 leaders likely to be more or less known : in some cases 

 perhaps both known and objected to. 



The optical fractions of Sir George Stokes, commonly 

 written h c e f, are defined, as everyone knows, as follows. 

 A ray falling upon a denser body with 

 incident amplitude 1 yields a reflected 

 amplitude h and a transmitted c. A 

 ray falling upon the boundary of a 

 rare body with incident amplitude 1 

 has an internally reflected amplitude e 

 and an emergent /. General prin- 

 ciples of reversibility show that 

 b-\-e=0, and that b 2 + cf= 1 in a trans- 

 parent medium. 



Now in our present case we are attending to perpendicular 

 incidence only, and we are treating of a conducting slab; 

 indeed, we propose to consider the obstructive power of the 

 material of the slab so great that we need not suppose that 

 any appreciable fraction of light reflected at the second surface 

 returns to complicate matters at the first surface. This limita- 

 tion by no means holds in Mr. Heaviside's complete theory, of 

 course, but 1 am taking a simple case. 



