278 Prof. R. W. Wood on the Existence of 
method is to use a thin prism, and cover the slit except for 
a small portion immediately above and below the single 
dark band of extinction. 
With this arrangement of the apparatus the magnetically 
rotated line should penetrate the dark band from above or 
below, according to whether the rotation is positive or nega- 
tive. If we excite the magnet and gradually heat the sodium 
tube, we see sharp needles of light shoot down from the 
continuous spectrum into the dark region immediately to 
the right and left of the D lines, as has been described by 
Macaluso and Corbino, Zeeman, and others. If we reverse 
the magnetic field the needles of light shoot up from below. 
The direction in which the plane of polarization is rotated 
by the D lines indicates that they are caused by vibrations of 
negative electrons. The important question to be answered 
is whether the absorption-lines of the band-spectra rotate the 
plane of polarization in the same or in the opposite direction, 
and whether they all behave alike. 
The magnetic-rotation spectrum being much brighter in 
the red and orange than in the green and blue region, the 
first observations were made in this part of the spectrum. 
The spectroscope was a medium-sized instrument, consisting 
of a telescope and collimator of about 180 cms. focus, 
furnished with a plane grating. 
The sodium tube was heated until the fine black absorption- 
lines in the red appeared distinctly in the continuous spectrum 
above and below the horizontal dark band due to the Fresnel 
prism. The current was then thrown into the magnet, the 
self-induction of which is so great that the field does not 
rise to its full intensity for several seconds, so that there was 
plenty of time to see exactly what happened. As soon as 
the switch was closed numerous needles of light commenced 
to penetrate the dark region, some of them shooting down 
from above, others shooting up from below. Of these, some 
only extended halfway or less across the dark band, while 
others crossed it completely. On opening the switch the 
luminous needles slowly withdrew from the dark background 
into the bright region from which they came, reminding one 
of the tentacles of an alarmed hydroid. The phenomenon is 
one of the most beautiful that I have seen for some time, 
for it shows us at once that some of the absorption-lines 
rotate the plane of polarization in the positive direction, 
while others rotate it negatively. 
A very satisfactory photograph of the phenomenon was 
obtained on a Wratten and Wainwright panchromatic plate 
