﻿On the Cosine Law of Radiation. 237 



the ground. On the other hand, many observers have found 

 aurorye, especially rays and ray-bundles, to reach within a 

 distance of one km. If so, the rays must possess an enormous 

 penetrating power, and yet the straight-lined structure will 

 make a radiation of the a-ray type necessary ; but it is a 

 type not yet known from laboratory experiments. 



It is a remarkable fact that the rays and ray-bundles often 

 shoot down in between other forms of aurora which do not 

 reach so near to the earth. In view of our radiation hypo- 

 thesis, this would mean that these very penetrating rays had 

 about the same stiffness as ordinary a-rays, or 



mv m v , , , v 



— = — r-^ (approximately), 

 ne le 



w T here m and v are respectively the mass and velocity of the 

 solar rays and ne the charge of each particle ; m , v , and 

 "le are the corresponding quantities for a-rays. 



From what we know about the charge carried by the small 

 ions and by the positive rays in vacuum-tubes, n ought to be 

 a very small number. Then in order to explain the great 

 penetrating power of the solar rays, we should have to suppose 

 a carrier smaller than a helium atom, but moving with a 

 velocity so much greater than that of a-rays that the above 

 relation is fulfilled. The carrier might either be hydrogen 

 or some gas lighter than hydrogen, and the possibility 

 suggests itself that coronium may form the carrier of the 

 most penetrating solar rays. 



XIX. JSote on the Cosine Law of Radiation. By Louis 

 Vessot King, B. A. {Cantab.), Lecturer in Physics, McGill 

 University *. 



§ 1. IT is a well-established fact in the theory of light that 

 JL the intensity in a parallel beam of homogeneous 

 radiation travelling a distance x through an absorbing medium 

 is diminished according to the exponential law 



I = V-*, (i) 



where I is the intensity over unit cross-section measured at 

 an arbitrary origin x = 0. k is a constant for the medium, 

 called the coefficient of absorption, and depends on the wave- 

 length of the radiation. 



If the radiation emanate from a point-source ,«^vp consider 



* Communicated by Trot*. H. T. Barnes, F.R.S. 



