DECEMPER 13, 1906| 
Double Refraction and Resolution of the Absorption Lines. 
In the second place, we will now consider the double 
refraction which occurs whenever light traverses a vapour 
at right angles to the magnetic field. A plane wave with 
vibrations parallel to the field has a velocity different from 
that of a wave with vibrations at right angles to the field. 
It is only close to the absorption band that the difference 
becomes perceptible. Sodium vapour in a magnetic field 
behaves as a double refracting crystal for light close to 
the sodium lines. This result .of Voigt’s theory was 
verified by him in conjunction with Wiechert in the case 
of dense vapours, and commented upon by Becquerel and 
Cotton. 
With great density, and using the same system of inter- 
ference bands, the phenomenon assumes the appearance 
now projected. Whereas the rotation of the plane of polar- 
isation was symmetrical on both sides of the absorption 
band, you see that the double refraction is not. On one 
side of the absorption line sodium vapour behaves like a 
positive crystal, on the other side like a negative one. 
With very dilute sodium vapour, and with a magnetic 
field strong enough to resolve the sodium lines, the theory 
must be extended. There is no difficulty here. The 
observations made by Mr. Geest, as well as by myself, 
concerning the details of this double refraction, have fully 
confirmed Voigt’s theory.* 
The slides shown always refer to one of the yellow 
sodium lines, and hence the structure seen is almost 
entirely confined to the extremely small region between the 
components of one line. The line D, splits up into three 
components in a moderate field. The theoretical course of 
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Fic. 5. 
double refraction is given in a diagram; next to it the | 
result of observations is given (Figs. 6 and 7). On a 
somewhat larger scale the appearance is as now shown; 
with greater density the characteristic sinuous line under- 
goes transformation. The line D, splits up into a quartet. 
Besides the concave parts, you will now notice a line with 
a point of inflexion in the theoretical and in the observed | 
curves. 
The same phenomenon is again illustrated by the next 
slide, where also the change which occurs with greater 
density is manifest. In a very strong field the line D, is 
resolved into a sextet. The inverse sextet can be readily 
seen with the means at our disposal, but the phenomena 
occurring between these narrow-spaced components could | 
only be seen with difficulty. Only in very favourable 
circumstances Mr. Geest observed the image now projected. 
All the described phenomena are qualitatively in excel- 
lent accordance with Voigt’s theory. It is certainly very | 
interesting that the theory is able to explain the compli- 
cated course of double refraction by the difference between 
the velocities of propagation of vibrations at right angles 
and parallel to the field. 
Magnetic Resolution and Intensity of Field. | 
Let me again refer to our first subject, the magnetic | 
separation of the lines. The magnitude of this separation 
is proportional to the intensity of the field in which the 
source is placed. We may, therefore, deduce the intensity 
of the field from the magnitude of the magnetic separ- 
INSTA S NOIRE: 
1 Zeeman and Geest, Proc. Acad. of Sciences, Amsterdam, May, 1903, | 
December, 1g04. Geest, Thesis, Amsterdam, 1904, Archiv Néerl, sér. 2, | 
T. 10, p. 291, 1905. | 
NO. 1937, VOL. 75 | 
161 
ation. We have only to measure the distance of the com- 
ponents of a suitable line. It is not generally known that 
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this distance can be measured with great accuracy (with 
an error of considerably less than 1 per cent.). It is, 
therefore, far easier, if a re- 
latively high degree of accuracy 
is necessary, to compare the in- 
tensities of field by measurements 
of the distance between the com- 
ponents than by direct magnetic 
measurements. 
All methods used for the 
measurement of magnetic fields 
give us the intensity in a point. 
On the other hand, the magnetic 
resolution of spectroscopic lines 
can give us the intensity in all 
points belonging to a_ line. 
Moreover, in this manner we 
make direct use of a property of 
the atom. 
You see here a vacuum tube 
We heat the tube and excite it with 
with some mercury. 
which how- 
the coil. You notice the brilliant light, 
ever, greatly increased when the tube is 
placed in a magnetic field.t For a given 
density of the vapour there is a definite 
intensity of field for which the luminosity 
a maximum. You can see this when 
we put on the current in the electro- 
magnet; the intensity of the field then 
rises gradually. 
We project an image of the tube on the 
slit of a spectroscope. This spectroscope 
must be so arranged that to every point 
of the slit there corresponds a point of 
the image. The blue line of mercury 
(4359) resolves into a sextet. Using this 
line, the field of a du Bois electromagnet 
with a pole distance of 4 mm. is mapped 
out in the spindle-shaped optical magneto- 
grams now shown (Fig. 8). We may, of 
course, extinguish the light of the inner 
components. In some cases a triplet will 
give more accurate results. The method 
sketched will, of course, only be applied 
in difficult cases. So long as our spectro- 
scopes of great resolving power are rather 
cumbersome there is no practical appli- 
cation for the method. By means of this 
method we may also study some questions as to the way 
in which certain phenomena which accompany the resolu- 
tion depend ‘on intensity of field. 
1 Paschen, ‘‘ Physik. Zeitschr.,” I. S., 478, 1900. 
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