504 
PROFESSOR OLEEK MAXWELL ON THE ELECTROMAGNETIC FIELD. 
Relation between Electric Resistance and Transparency. 
(106) If the medium, instead of being a perfect insulator, is a conductor whose resist- 
ance per unit of volume is g>, then there will be not only electric displacements, but true 
currents of conduction in which electrical energy is transformed into heat, and the undu- 
lation is thereby weakened. To determine the coefficient of absorption, let us investi- 
gate the propagation along the axis of x of the transverse disturbance G. 
By the former equations 
^ = -4 Tyj(q') 
d * G . /I d^G 1 rfG\ , .-p,. , /nc:\ 
^ = + 4^ w -- by(E)and(F) (95) 
If G is of the form 
G=e~ p * cos (qx-\-nt), (96) 
we find that 
f 97) 
g q g i v 
where V is the velocity of light in air, and i is the index of refraction. The proportion 
of incident light transmitted through the thickness x is 
(98) 
Let R be the resistance in electromagnetic measure of a plate of the substance whose 
thickness is x, breadth b, and length l, then 

(107) Most transparent solid bodies are good insulators, whereas all good conductors 
are very opaque. 
Electrolytes allow a current to pass easily and yet are often very transparent. We 
may suppose, however, that in the rapidly alternating vibrations of light, the electro- 
motive forces act for so short a time that they are unable to effect a complete separation 
between the particles in combination, so that when the force is reversed the particles 
oscillate into their former position without loss of energy. 
Gold, silver, and platinum are good conductors, and yet when reduced to sufficiently 
thin plates they allow light to pass through them. If the resistance of gold is the same 
for electromotive forces of short period as for those with which we make experiments, 
the amount of light which passes through a piece of gold-leaf, of which the resistance 
was determined by Mr. C. Hockin, would be only 10 -50 of the incident light, a totally 
imperceptible quantity. I find that between and x^o“o °f green light gets through 
