( 170 ) 
p. 281) illumination is necessary to . excite permanent emission to 
the full. If the illumination has not been sufficiently prolonged, or 
has been performed with a spectrum that was not sufficiently intense, 
it can happen that places of moderate duration will at first show a 
clearer afterglow than places of maximum duration because the latter 
had not yet been sufficiently excited. 
As has already been said, the momentary part always seemed to 
overlap the permanent part, sought for in this way. The former 
can be observed during the first illumination with the spectrum, or 
better still, in that part of the phosphorescent light which quickly 
disappears immediately after the illumination ceases. The momentary 
glow that was to be observed in these parts of the momentary 
portion, e.g. in the extreme violet, showed in all cases the character 
of the upper momentary phase, that is to say, there was no storing 
of electrons; parts illuminated by this portion of the spectrum cannot 
be made to glow either by warming or by ultra-red excitation, only 
those parts of the spectrum which belong to the permanent part 
will induce a glow. 
There are thus definite wave-lengths which in all temperature 
phases of the centra bring into action the momentary process ex¬ 
clusively, or almost so; that is to say, the electrons that under the 
photoelectric influence of these wave-lengths are ejected from the 
metallic atoms of the centra return quickly to their metallic atoms, 
giving out light, while at the same time other wave-lengths cause 
electrons to be ejected from the metallic atoms of the same kind of 
centra, and these electrons are retained in the neighbourhood (storing, 
process of atterglowing). This circumstance can he ascribed to the 
fact that the ejection from the metallic atoms caused by the various 
wave-lengths takes place in different directions in space (fired in the 
atom), and therefore also in different directions with regard to the 
neighbouring atoms of the centrum, the atoms of the alkaline earth 
metal, the sulphur atoms, perhaps also atoms coming from the 
additional substance. 
Only those electrons which are directed towards places in the 
neighbourhood suitable for retaining electrons, are retained and stored, 
thus causing the permanent phase as the temperature falls within 
the region of permanency. It has already been indicated where these 
places must be looked for whose presence makes storing possible. 
It has already been shown in “1904” that they can only be place 8 
within that molecule to which the name “centrum” has been given* 
“For every band (kind of centrum) even of the same “phosphor” 
stores (“1904” p. 473 et seq. p. 481) up its own excitation m 
