808 
transmission for the mentioned wavelength. We define the latter by 
superposing each spectrum by a He-spectrum. The exposition-time 
must be the same for all the spectra. The method used has the 
advantage that the absorption for all colours is measured at the 
same time, and that it is produced in all spectral areas under identie 
circumstances. 
2. We shall now further expose the details of our method. In 
the first place we want to illustrate the use of the screens and to 
indicate the method for measuring their faculty of transmission for 
different colours. From the blackening, measured on the photographie 
plate, the intensity of the incident light is generally calculated 
according to the approximation-formula of ScHWARZSCHILD 
z = blackening of the photographic plate 
z=log.].t? ¢ |] =intensity of the incident light 
t = exposition-time 
wherein p is a constant varying from plate to plate. In order to 
avoid the use of such an approximation, we worked out a method, 
which renders it possible, without using this law, to find the relation 
between the intensity of the incident light and the blackening which 
it produces on the photographic plate. 
This relation can be found for each plate separately in an ex- 
perimental way by constructing a blackening-scale on each plate. This 
scale is obtained by means of a series of spectra, which are reduced 
in a known degree; in order to reduce these spectra several screens 
are put in the way of the rays. The screens are subjected to the. 
following conditions : 
1st. they have to absorb the light for all the waves to the same 
degree, that means: not to show any selective absorption; or — in 
case they have got any — the absorption for each colour must be 
easily determinable; | 
2rd, the structure of the screens has to be so subtle that its image 
on the slit of the spectrograph does not disturb the regularity of 
the photographed spectrum *); 
3'4. finally it must be possible to get the screens in any desired 
degree of transmission. 
A uniformly blackened photographic plate fulfils all these con- 
ditions. A first experiment showed that it does not possess any 
1) Reducing the light-intensity by nicols turned out to be too inaccurate. A 
tissue disturbs the regularity of the spectrum. 
