( 587 ) 
With very diluted vapour no absorption at all or only very weak 
traces of absorption are seen. 
4. The introduction of a Nicol in the beam before or after the 
field entirely changes the phenomenon. The absorption lines can then 
be seen very narrow and black. 
Let the observation be made at right angles to the horizontal 
field, then, if the Nicol is placed with its plane of vibration vertical 
D, exhibits its two, D a its four outer components. 
After a rotation of the Nicol over 90° both D, and D s give only 
the two horizontally vibrating components. 
Let a beam of natural white light traverse axially the magnetized 
vapour placed between the perforated poles of an electromagnet. 
Then by means of a quarter-wave plate and a Nicol we may quench 
either the right-handed or the left-handed circularly polarized 
component. 
A combination of a quarter-wave plate and a Nicol, converting 
incident light into right-handed circularly polarized light may be 
called a right-handed circular analyser. The absorption line corre¬ 
sponding to a right-handed circularly polarized component is seen 
with both increased clearness and darkness by examining it with a 
right-handed circular analyser. 
We introduce here this simple matter because there has been 
occasionally some confusion on this subject. 
5. The behaviour of horizontal and vertical vibrations may be 
studied simultaneously by using according to the - suggestion of 
Cornu and Konig a calcspar rhomb. By means of it we can 
obtain two oppositely polarized images of a horizontal slit of suitable 
width, placed near the magnetic field. 
Right-handed and left-handed circular vibrations can be separated 
on the same plan by the introduction of a Fresnel rhomb between 
the calcspar and the slit of the spectroscope. 
It is, however, of considerable interest to examine also the behaviour 
of the lines in natural light. A separate examination after the removal 
of the polarizers might be made. The vapour density ought to be 
the same in both experiments. It seems difficult to realise this. 
The desired end is secured more simply and surely, and with 
only half the labour, by adopting the width of the horizontal slit and 
the thickness of the calcspar in such a manner that the two images 
given by the calcspar partially overlap. We now obtain three stripes; 
the central one exhibits the phenomena as seen without polarizing 
apparatus. (See fig. 2). 
