( 590 ) 
by this method. A small disturbance is produced by the narrow 
reversed lines due to the electric arc light. The intensity of these lines 
depends upon somewhat variable circumstances of the arc itself. In 
some cases these lines are almost invisible, in other ones more prominent. 
They are to be seen on some of our reproductions; with our present 
subject they have nothing to do. 
As regards the central stripe we refer to the remark previously 
made, that the image of the separation must become, on account of 
the only partial absorption, rather indefinite and weak. 3). 
The partial superposition of components gives, at least in the case 
of diluted vapour, the most conspicuous lines. (§§ 6 and 7). 
In the case of the quartet, for example, one sometimes sees instead 
of four, only two components, situated between the inner and outer ones. 
We made experiments with different vapour densities. The observed 
phenomena may be classified under three phases: 
1. The vapour is very dilute. The components are clearly visible 
in the upmost and lowest stripe. In the central stripe the absorption is 
either hardly perceptible (Plate I, Fig. 1) or the components of the 
quartet and the sextet are seen as separate, but weak lines. (Plate I, 
Fig. 2). 
In this phase of the phenomenon the great difference of definiteness 
of the central and outer regions is very remarkable. This contrast 
is still more marked with eye observation. 
In order to obtain good photographs, it was necessary to increase 
the density of the vapour above the one required for the observation 
of the very first trace of absorption. 
2. Vapour of intermediate density. 
The components in the upmost. and lowest stripes are now no more 
separately visible or only in the case of the quartet. In the central 
stripe a superposition of the kind mentioned in § 6 takes place. In 
place of the quartet an apparent doublet is seen, the components of 
which are situated between the outer and inner components of the 
quartet. This case is very clearly represented in Plate I, Fig. 3. 
The phenomena exhibited by the sextet (D s line) become rather 
complicated. 
The superposition phenomenon is often very distinct. The D 2 line 
on Plate I, Fig. 3 shows sufficiently the appearance. 
3. With still denser vapour, the components become very broad 
and the magnetic change hardly visible. The polarisation of the edges 
of the broad line may be recognized. This phase is represented in 
Plate I, Fig. 4. It corresponds to the emission effect as it was first 
discovered: a slight change of broad lines in a weak field. 
