TEMPERATURE AND STRUCTURE OE THE SUN. 89 
rp 
l max- 
rn . 
1 'nun* 
Arc lamp 
Nernst lamp 
WelsbacK mantle 
Incandescent lamp. . . . .... 
Candle 
Gas flame 
0,7 m 
1,2 
1,2 
1.4 
1.5 
1,55 
4200° Abs. 
2450 
2450 
2100 
1960 
1900 
3750° Abs. 
2200 
2200 
1875 
1750 
1700 
Furthermore, having thus determined the temperature of 
a light source, we can use the spectral equation for calculating 
the distribution of energy of black radiation at that temper- 
ature and compare it with the distribution curve observed. 
In Figure 2 the dotted line represents the calculated dis- 
tribution and the heavy line the observed curve. By such 
experiments we see that all the substances radiating in our 
practical sources of light are, like the sun, far from being 
“ideal” light radiators, which I defined to be black bodies 
with respect to the visible spectrum and perfect mirrors with 
respect to the invisible spectrum. Is it possible that in nature 
such ideal light radiators may exist? I do not know, but 
if they do exist it is possible to raise the economy of light 
production to 10 or 100 times that attained at present from 
the incandescent lamp, candle, etc. The economy might 
thus be increased by as much as the total radiant energy in 
the invisible spectrum surpasses that within the visible spec- 
