Vol,. 6, 1920 PHYSICS: L. PAGE 115 
reaction in which a photosensitive substance is broken down into two 
decomposition products. During dark adaptation of the retina these 
two decomposition products then unite to form again the sensitive sub- 
stance from which they were produced. The actual course of dark adapta- 
tion depends on the change in concentration of these two reacting sub- 
stances. 
In all essentials, then, the mechanism underlying the initial phase of 
retinal sensitivity in dim light is the same as that which forms the basis 
of the initial process of photoreception in such forms as Mya and Ciona. 
^ W. Nagel, in Helmholtz' Handbuch der physiologischen Optik, 2, 1911. 
2 S. Hecht, Journal of General Physiology, I and 2, 1918-20. 
A KINEMATICAL INTERPRETATION OF 
ELECTROMAGNETISM 
By IvEigh Pag^ 
S1.0ANE Laboratory, Yai.e University 
Communicated by H. A. Bumstead. Read before the Academy, November 10, 1919 
The concept of lines of force was introduced by Faraday as an aid in 
mapping out magnetic fields — these lines having everywhere the direction 
in which a small north pole would be urged by the field. Later it occurred 
to him that the characteristics of an electric field might be represented in 
similar manner by curves everywhere tangent to the electric intensity. 
Faraday limited his use of lines of force to static fields, and in cases where 
the field was produced by a number of charged particles, to the resultant 
field. 
The electromagnetic theory of light, however, made it necessary to 
suppose that electric and magnetic lines of force in a wave front move with 
the velocity of light in a direction at right angles to their plane. Further- 
more, the discovery of the electron made it natural to assume that when 
one of these small particles is in motion, its field is carried along with it. 
Wiechert and Stokes have explained X-rays as kinks in the lines of force 
emanating from such a particle — these kinks being propagated outward 
with the velocity of light whenever there is a change in the electron's 
state of motion. 
The object of this paper is to show that the laws of electromagnetism 
may be explained exactly and in their entirety as kinematical relations 
between the moving elements which constitute lines of force, provided 
use is made of the space and time transformations of the principle of rela- 
tivity in place of the more familiar but less philosophical Galilean trans- 
formations.^ Each charged particle must be considered to have its own 
field, extending out to infinity in all directions, the resultant field at any 
point being a combination of those fields which extend to the point in 
question. 
