558 
Proceedings of the Royal Society of Edinburgh. [Sess. 
An example will make the method clear. When a tantalum lamp was 
run at its marked voltage, 125 volts, the fraction of the total radiation 
transmitted by the water cell was 0*222. The energy curve is given 
below, the depression to the left of the top being due to an absorption 
band of water. 
The fraction of the area on the left of AB is 0*251. Hence the radiant 
efficiency is 
*251 x *222 
= b 7 per cent. 
*84 
o 
In comparing the energy-curve method with Angstrom’s method, 
Nichols and Coblentz give the preference to the latter on account of the 
errors introduced in reducing the prismatic energy curves to normal energy 
curves. In finding the ratio of the areas it is, however, not necessary that 
the abscissse should be wave-lengths. If s denote the spectrometer reading 
and h the galvanometer throw, to change to the normal spectrum we 
cLs ds 
multiply h by ^ • The normal energy spectrum is plotted as a func- 
f ds f 
tion of X. Its area is given by | h-^dX, which is the same as J hds. 
