798 
vanometer with reducer before the thermopile, by that without 
reducer. By placing a concave lens before the reducers it was possible 
to modify the convergence of the beam of light. It appeared to have 
no influence within wide limits. 
Five reducers were used, which transmitted resp. 68.0, 46.8, 32,5, 
21,3 and 14.7 °/, of the incident light. 
The blackenings were determined with Morr’s microphotometer *). 
In the registered curves belonging to the unweakened system of 
lines and to the weakened system of lines obtained in the way 
described above, the maximum of one of the lines, e.g. of the 
intensest of the system, was found. The blackening of this point 
was plotted for the different spectra with respect to the intensity, 
— blackening. 
~D ee ec ' 
147 21,3 32.5 468 68,0 100 
— log I 
Fig. 2. 
in which, as is usual, the abscissa was taken proportional not to the 
intensity 7 itself, but to log 7. This gave the curve AB (fig. 2), in 
which the intensity of the considered line in the unweakened spectrum 
is put arbitrarily at 100, so that the intensities of the other five 
points have the values mentioned above. 
If for another (fainter) line we put the intensity in the unweakened 
spectrum again at 100, we get for this line the curve CD. The 
1) Verslag Kon. Akad. 27, 566, 1919. 
