Measurement of High Temperatures. 45 
bodies which absorb all colors in equal proportion. They - 
appear colorless when nearly transparent, white or gray when 
opaque, according to the intensity of the light falling upon 
them and to their reflecting power. That many transparent 
bodies belong to this class and not to class I, is evident from 
the fact that although they remain transparent and colorless at 
temperatures above that at which metals are red hot, it is possi- 
ble by heating them still further to cause them to glow brightly. 
Such a change in the power of emission corresponds to an in- 
crease in absorptive capacity. 
That in general the absorptive capacity of other than black 
bodies cannot be a constant quantity may be inferred from the 
usual equations for the intensity. of the reflected ray. Let the 
body in question be opaque. Of all rays falling upon it one 
portion will be reflected, the remainder absorbed. It has how- 
ever been proved that such bodies are in general transparent 
when taken in sufficiently thin layers. The rays must there- 
fore instead of being converted into heat at the surface, force 
their way to a certain depth into the interior of the body, and 
we are justified in assuming that refraction occurs. Let 7 be 
the angle of refraction, and 7 the angle of incidence of a certain 
pencil of light falling upon the substance. Let the intensity 
of the incident ray =1. Whatever the character of its vibra- 
tions—provided only that in accordance with the accepted 
theory they be transversal—the ray can be resolved into two 
components, the one polarized in the plane of incidence, the 
other perpendicularly to it. Let e, and e, be the amplitudes of 
these components the intensities of which are denoted by /, 
and 2 en 
Fae i ae 
a es of the reflected portion of the component e, 
will be 
_. sin(t—r) 
: B= sin (ir) (2) 
and its intensity, 
sin *(¢—r) 
Be sin (ihr) x 
The amplitude of the other part of the reflected ray will be 
_ tang (¢—r) 
| B= ang (Fr) “ 
and its intensity 
__,- tang *(é—r) 
R= tang *(i-++-r)" (5) 
The expressions (8) and (5) approach 0 as a limit when the 
values of r and 7 are made to approach each other. In other 
words, when the optically denser medium becomes less dense 
