849 
The investigation of the blackening for different colours has, besides, 
another practical use. Blackened photographic plates have often served 
as light-reducers. Of late they have been used as such in different 
researches in the Institute for Theoretical Physics. Compare Miss 
RIw1in’s investigation *). It was, therefore, necessary to find a method 
to gauge these reducers for different colours with regard to the 
quantity of light that they allow to pass. 
The attention may also be drawn to the fact that the law of 
blackening that is found, can present differences when the plate is 
measured for different colours. 
For the blackening is dependent on the method by which it is 
measured, in opposition to the number, the section, and the nature 
of the grains of silver present in the photographic plate per unit of area. 
The micro-photometers work with light of different colours, thus 
e.g. that of Hartmann’) and all other visual ones chiefly with 
yellow-green, that of Paur, Kocu ®) chiefly with light of short wave- 
lengths, that of Morr *) for the greater part with red and ultra-red. 
Differences in results may, therefore, always be interpreted by the 
consideration that the blackened photographic plate is far from black. 
$ 1. Determination of the Blackening for Different Parts 
of the Spectrum. 
1. Method of procedure. The determination of the blackening 
takes place by means of the extinction meter of Morr, modified 
according to the adjoined scheme. °) (fig. 1). 
A Nitralamp Lp (25 candles, 4+ Volts) throws a beam of light 
made parallel’) by lens Ls on the photographie plate Pt, which is 
always put at the same place in the apparatus. A dish Ct filled 
with a coloured liquid, and a colour-filter #7 enable us to throw 
a beam of the desired colour on the plate. After this the beam 
strikes the thermopile 7 and causes a thermo-current proportional 
to the energy of the light that strikes it. The intensity is measured 
by the aid of a compensation method. ; 
1) These Proc. Vol. XXIII N° 5. p. 807. 
*) Zeitschr. f. Instrum. Kunde 1899 p. 97. 
4) Ann. der Physik Bd. 39, 1912, p. 705—751. 
4) Dr. W. J. H. Morr. Een nieuwe registreerende microfotometer. Versl. 
Kon. A. v. W. XXVIII (1919), p. 566. 
5) Versl. Kon. Ak. v. W., 28, p. 1001—1006. 
6) We choose a parallel beam to make the theory of the observed phenomenon 
as simple as possible. We avoid then also the Callier effect (Zeitschr. f. Wiss. 
Photabd: e75-p. 257 etc). 
