Energy Intensity. 
42 Ingvar Jorgensen and Walter Stiles. 
blue light of the same energy. This problem has been attacked 
by Kniep and Minder (1909). They used a Rubens’ thermopile for 
the measurement of radiant energy. Light of different colours were 
obtained by the use of different filters. 
For red a glass filter was used which let through light of wave 
lengths 620/x/x —infra red, and a little light of wave length 608/z/x. 
The coefficients of transmission given for different wave lengths 
were— 
Wave Length. 
Coefficient of Transmission. 
ix,x 
644 
578 
546 
509 
0-846 
0-00056 
0-000057 
0000 
The blue filter let through light of wave lengths 523-8/x/x—ultra 
violet. The coefficients of transmission given for different wave 
lengths were— 
Wave Length. Coefficient of Transmission. 
uu 
Fig. 18. Energy Intensity in the Normal Spectrum of direct Sunlight 
after Langley (Curve A), compared with the Energy Intensity in the Spectra 
of Sunlight after its passage through Red (Curve R) and Blue (Curve B) Filters. 
The broken part of Curve R is assumed. (After Kniep and Minder). 
