( 586 ) 
field ; the doublet type if the light is examined parallel to the lines 
of force. D, seems to exhibit a doublet in both principal directions. 
The Fraunhofer lines in the spectra of sun-spots investigated by 
Hale are either broadened, or changed to doublets (often incom¬ 
pletely resolved quartets), or to triplets. The resolutions exhibited 
by sodium vapour are therefore the very types of special importance 
to astrophysics; this and also the facility of producing sodium 
vapour in the magnetic field induced us to commence our experiments 
with this substance. 
3. The explanation of the inverse effect is easily understood by 
means of the well known law of resonance. If there are in a 
flame under the influence of a magnetic field three periods of free 
vibrations, then we may expect that from incident white light vibra¬ 
tions of these very three periods will be taken away. The absorption 
is a selective one, with this peculiarity that the selection refers not 
only to the period but also to the direction of vibration. Consider 
for example the central component of a triplet which in the emission 
spectrum is due to vibrations parallel to the field. From incident 
white light only vibrations, corresponding as to period as well as to 
direction of vibration with the middle component, are absorbed. 
Vibrations, perpendicular to the field, though of the period of the 
unmodified line, pass unimpeded. 
On the contrary white light of periods coinciding with those of 
the outer components is only deprived of its vertical constituents. 
It will be clear from these very simple considerations what we 
may expect to observe with white light under the conditions of the 
experiment. The arrangement was the following: White light of the 
incandescent positive pole of an arc-lamp traverses a sodium flame, 
placed between the poles of a du Bois-electromagnet. This light is 
analysed by means of a stigmatic spectroscope with large Rowland 
grating. The observations are made in the first order. 
If the observation is made at right angles to the lines of force, 
we see in the continuous spectrum 4 dark components in the case 
of Z>j, 6 dark components in the case of I) 2 , as represented for both 
lines under a in the diagrammatical Figure 1. 
In order to observe all these components the field must be strong 
and the vapour density adapted to the field. 
The groups of lines indicated by b are seen, if the light is examined 
axially. 
All these components, if narrow, are seen only diffuse and not black. 
From the considerations above given the reason will be clear at once; 
each of the components absorbs only half the incident natural light. 
