809 
absorption of importance in the visible sphere.*) This preliminary 
result gave rise to an extensive research of Mr. A. Drumens, which 
will be published in these proceedings. The plate proved to show a 
selective absorption, which may be greatly reduced under suitably 
chosen circumstances. The screens were therefore measured again 
by means of the method elaborated by Mr. Droumens, and at the 
discussion of our experiments the decrease in every region was _ 
observed. We made use of about ten photographic screens, whose 
faculty of transmission in, percentages were chosen according to a 
mathematic progression. In this way the most accurate results can 
be obtained; as the blackening is approximately proportional to the 
logarithm of the intensity, we obtain with this series of screens a 
regularly increasing table of blackenings. 
3. The blackening {of the photographed spectra was measured 
with the photometer of Dr. W. J. H. Motu’). We used the small 
and simple apparatus of the Institute for Theoretical Physics, which 
is suitable to determine the blackening for extensive spectral regions. 
The apparatus differs in some respects from the microphotometer 
described in these Proceedings by Dr. W. J. H. Morr; viz.: 
1st. The slit S, is left out (see l.c. p. 571, fig. 5) while the 
microscopic objective Q, produces an image of the incandescent 
spiral on the plate P; 
2d, The velocities of the photographie plate P and of the 
registration-cylinder A were regulated in such a way that a removal 
of the photographic plate over 10 cm. corresponded to a removal 
of the sensitive paper on the cylinder over half its length i.e. 
about 20 cm. 
Before and after each spectrum a piece of the clear unblackened 
plate passed through the way of the rays, and the deviation of the 
galvanometer reached its maximum. 
The dark Heliumlines, which are marked as notches in the un 
1) This first experiment showed the following result: 
Colour of the incident light. Faculty of transmission of the screen. 
Red 27.4 4 
Yellow 27.6 0/, 
Green 27.0 0/, 
Blue 25.6 "/o 
Violet 25.0 0/, 
Total visible spectrum 27.20/,. 
2) Dr. W. J. H. Morr, Een nieuwe registreerende microfotometer. Versl. Kon. 
A. v. W. XXVIII (1919), p. 566. 
