( 9 
Big. 1. Fie. 
bo 
Fig. 3. 
Increasing now the quantity of sodium (always remaining very 
small however, absolutely) the interference fringes moved upward 
along the components of the doublet, whereas the part of the fringe 
between the components seemed no longer connected to the exterior 
fringes and accepted the shape figured schematically in fig. 2. 
Increasing still further the density of the vapour the interior part 
of the fringe slid downward with increasing velocity and then 
resembled an arrow with point directed upward, the parts more 
removed from the medium line fading away and disappearing (cf, the 
schematic fig. 3). At last the arrow entirely disappeared by the 
increase of the density of the vapour. It then became impossible to 
distinguish the fringes or any trace of structure in the field between 
the components. Rather much light was transmitted. The entire 
width of the components of the doublet was now about of the same 
order as the distance of their central lines. ; 
A further increase of the quantity of sodium obscured the central 
part more and more (see further (8)). 
The exterior fringes moved continuously upward while the density 
was being increased. 
In a field of about 20000 units the downward displacement could 
be followed over a distance of more than the double of the distance 
between two. fringes, corresponding to a negative rotation of above 
2 x 180°, say 400°. The distance between the poles was 4 mM. 
Some more accurate data will be given on another occasion. 
In the case of D, the phenomena were in the main of the same 
character. 
For D, it was however characteristic that the stage of the nearly 
or entirely vanishing of the interior fringes was reached with smaller 
field, whereas also the shape of the interior fringe differed from the 
one observed in the case of D,. Hence there exists also in this case a 
difference between J), and D,, a difference already known to exist 
in the phenomena of reversal, of the separation by a magnetic field 
and of the rotation of the plane of polarisation in the vicinity of 
the absorption band. 
