Theory of Radiation 
35 
39, 40] 
Stellar Radiation. 
40 . Of even greater importance than the luminosity, or quantity of light 
emitted by a star, is the quality of this light, as revealed by analysis in a 
spectroscope. 
Stefan found in 1879 that the amount of radiation emitted by a perfectly 
radiating surface of any kind is proportional to the fourth power of its absolute 
temperature T. The radiation per unit area of surface is usually taken to be 
crT 4 ergs per unit time, where a is “Stefan’s constant” whose value,according 
to the determinations of Coblentz* and Millikan f, is 
In 1893 Wien brought forward a general thermodynamical argument 
which shewed that the law of partition by wave-length must be of the form 
and in 1900 Planck discovered the form of the function f(\T), so that the 
complete law of radiation is now known in the form 
where G is the velocity of light, R is the universal gas-constant and h is 
another universal constant, “ Planck’s constant.” 
The values of these quantities are 
C= 2’998 x 10 10 cms. a second} 
The point of primary significance in these formulae is that they are entirely 
independent of the nature of the matter which emits the radiation, the con 
stants cr, h, R and C all being universal constants of nature. There is a simple 
physical reason for this. The radiation inside a hot body at a uniform tem 
perature T is absorbed and re-emitted many times before it reaches the 
surface, so that the radiation which finally emerges is in thermodynamical 
equilibrium with the matter of the body. Its constitution must thus depend 
solely on the temperature T. Stefan’s law may equally well be stated in the 
form that radiant energy in thermodynamical equilibrium with matter at 
temperature T is of amount aT 4 per unit volume, where a is an absolute 
constant of nature. This radiation, of course, travels equally in all directions. 
If it all travelled in the same direction, the amouut crossing a unit area of 
surface in unit time would be aCT*\ when allowance is made for the different 
directions of travel, the amount is found to be \aGT i . If this unit area forms 
part of the surface of a perfect radiator, this amount of energy \aGT* is equal 
* Phys. Rev. vn. (1916), p. 694; Sci. Papers of the Bureau of Standards, Nos. 357 and 360 (1920). 
t Phil. Mag. xxxiv. (1917), p. 16. 
a— 5 72 x IO -5 erg cm. -2 degree 4 
(401). 
E k dX = f(\T) A. -5 dX 
(40-2), 
(40 , 3). 
h = 6'55 x IO -27 erg seconds