( 168 ) 
§ 7. Momentary and permanent parts of the spectral excitation. 
There had still remained unsolved a question concerning the exci¬ 
tation-distribution in the spectrum which aroused our great interest, 
and in this respect a new result was obtained. 
A characteristic behaviour in the distribution of the exciting power 
in the case of the clear green head band « of the Sr Cu-“phosphors” 
had already been noticed (“1904” p. 470 • shown graphically by the 
curves 1 and 2 in fig. 2 and reproduced from PI. Ill N°. 21 loc. ch.)\ 
the distribution of the excitation over the spectrum did not disappear 
regularly, it separated into two spectra, the one quickly (momentarily) 
increasing and decreasing, the other of much longer duration over¬ 
lapping this, consisting of completely different groups, and slowly 
increasing and decreasing. By observing the gradual disappearance, 
and also the similar increase in intensity, one immediately gains the 
impression that here two different processes proceeding with different 
velocities overlap, each of which, however, has a different excitation 
spectrum (“1904”, p. 470). 
Then (although mentioning the other view that is now confirmed) 
the band was regarded as consisting of two coincident bands or of 
two bands lying very close to each other, a x 4- one of which at 
ordinary temperature should be in the momentary phase, and the 
other in the permanent phase, and which, as is always the case 
with different bands, should each have its own excitation-distribution 
over the spectrum. The same characteristic of the increasing and 
decreasing of the excitation-distribution occurring not as a whole as 
is the case with the Sr Cu a band, has later been mentioned by 
A. Werner 1 ) in the case of the SrZna band, and a more detailed 
investigation of a great number of Other bands repeated by P. Lknard 
with better apparatus 2 ) than those employed before showed that the 
above mentioned property is characteristic of all bands, and that « 
differs in the various bands only quantitatively. As well as these 
two bands, CaMn« ls BaCu«, CaBip are also striking examples of 
this. The great number of instances observed makes it improbable 
that in every case one is dealing with the overlapping of bands that 
lie very close together and are in- different temperature phases. 
The following observations for which different temperatures were 
chosen lead to the same conclusion. The observations at low temperatures 
were made according to the method already described ; for those 
*) A. Werner. Dissert. Kiel. July 1907. Ann. Phys. 24 p. 190, 1907. 
*) Wh ich were rendered available by the establishment of the Radiologic 
Institute at Heidelberg. 
