38 
The Light from the Stars [ch. ii 
If, however, we agree on a definite scale for X and E\, then different 
temperatures require different curves, and these lie as shewn in fig. 4. The 
four curves here shewn are drawn for the temperatures T— 3000, 4000, 5000 
and 6000 degrees absolute. These curves, which at a first glance look very 
different from one another, are all derived from the curve of fig. 3 by expan 
sion of its horizontal and vertical scales. 
When the temperature T is specified, formula (40'3), which is represented 
graphically in fig. 3, gives E\ as a function of X. The wave-length for which 
E K is a maximum is of course the value of X at the peak of the curve shewn 
in fig. 3; this is generally denoted by X raax . Formula (402) shews that, in 
general, X max must vary inversely as T, while by differentiation of formula 
(40'3) the actual relation is found to be 
X max . T= 0-2885 cm. degrees (431). 
The whole area of the curve shewn in fig. 3 represents the total radiation 
of energy at a given temperature distributed according to wave-length. The 
majority of this area is concentrated round the ordinate X = X max , so that to 
a rough approximation the radiation from a body at temperature T may be 
thought of as being all of the same wave-length X max , determined by equation 
(431), and therefore as being all of the same colour. In fact, we can arrange 
a scheme in which the colours of radiation are regarded as corresponding to 
the temperature of the body by which it is emitted as follows: 
Colour 
Wave-length 
Temperature 
Reddest visible light 
Yellow light 
Sunlight 
Blue light 
Most violet visible light 
X = 7500 A. 
6000 
5000 
4500 
3750 
2950 
T— 3850 degrees abs. 
4800 
5750 
6400 
7700 
9800 
Limit of atmospheric absorption ...