(591 ) 
With still greater absorption the influence of the field becomes 
imperceptible. 
All these phases appear with great regularity. If the intensity of 
the field is known, it seems possible, the resolving power of the 
spectroscope being given, to deduce the density of the vapour from 
the nature of the observed phenomena. 
The magnetic division phenomena hitherto observed in sun-spots 
appear to fall under the second and third phases above mentioned. 
Hale from measurements of spot lines, compared with laboratory 
experiments, deduces a maximum intensity of the spot field of 4500 
Gauss. Hence, one would be inclined to think that the density in 
the layers, which bring about the absorption in the sun-spot spectrum 
can only be small. Moreover, the non-uniformity of the field of 
sun-spots produces by itself a widening of the components. Light from 
a limited portion of the spot would give perhaps very narrow spectral 
lines. In the light, however, of the critical remarks of Kaiser *) 
concerning our knowledge of the influence of pressure and of tem¬ 
perature on spectra all such considerations most be put forward 
with great diffidence. 
11. Observations parallel to the lines of force. 
In the present experiments the absorbing vapour subjected to mag¬ 
netic forces is placed between perforated poles. 
After putting on the current, one sees in the continuous spectrum, 
2 dark bands in the case of If , 4 in the case of D ,, according to 
the diagrammatical figure 1. The absorption is incomplete also now, 
because of some wave-lengths only right-handed circularly polarized 
light, but not left-handed is absorbed and the reverse. In order to 
observe the separation and the polarization a Fresnel rhomb is placed 
with its principal plane at an azimuth of 45° with the horizon, a 
horizontal slit being placed in one of the perforated poles. The 
Fresnel rhomb converts circularly polarized into plane polarized 
light. By means of a calcspar rhomb also now three stripes are 
obtained! Thd first phase (very dilute vapour) is represented in Plate 
I, fig. 5. 
Vapour of intermediate density (second phase) exhibits the super¬ 
position phenomena mentioned in $ 7 and 8, and diagrammatically 
illustrated by Fig. 2. In the central strip one line, at the position 
of the unmodified one, surrounded by feebly absorbing regions, is 
*) Kayser. Haudbuch. Kapitel V. Bd 11. 
