Positive Electrons in the Sodium Atom. 275 
absorption spectrum of sodium vapour, have, as I have showD 
iu previous papers (Phil. Mag. Oct. 1905, Nov. iy06), the 
power of rotating the plane of polarization when the light is 
passed through the magnetized vapour in the direction of 
the lines of force. 
White light is passed through a nicol prism and a steel 
tube which passes through the pole-pieces of a large electro- 
magnet. An analysing nicol, condensing-lens, and spectro- 
scope follow in succession. The tube contains metallic 
sodium and is highly exhausted, for the vapour loses its 
rotating power when mixed with an inert gas. If we set 
the second nicol for extinction, the spectrum of the crater of 
the arc-lamp disappears^ but on heating the tube and exciting 
the magnet, a vast number of bright lines appear in the red 
and green-blue regions of the spectrum. Spectra obtained 
in this way j since they are radically different from spectra 
of other types, I have named " magnetic-rotation spectra." 
Macaluso and Corbino observed the effect at the D lines, 
employing a sodium flame between the poles of a magnet^ 
but they missed the complicated bright-line spectra which 
only appear when very dense sodium vapour is formed in 
vacuo. 
In the case of the rotation for wave-lengths in the vicinity 
of the D lines, there is no difficulty in determining the 
direction, i. e., whether positive or negative, for the broad 
bands of rotated light which border the absorption-lines can 
be moved from side to side by slight rotations of the analysing 
nicol : or we may employ the device so frequently used, the 
Fresnel double prism of right- and left-handed quartz, which 
tells us at a glance the direction of the rotation. In the 
case of the narrow lines of the channelled spectra, no in- 
formation can be gathered as to whether the rotation is 
positive or negative by rotating the analysing nicol, for the 
smallest possible turn from the position of extinction causes 
the continuous spectrum to brighten up, obliterating the 
rotation lines. It is, however, of the utmost importance to- 
determine the nature of the rotation in this case, as it will 
furnish many additional clues to the structure of the atom. 
An attempt was first made to employ metallic arcs in place 
of the white-hot crater, as the source of the light, on the 
chance that some of the lines might be of the right wave- 
length to suffer rotation in the region of the channelled 
spectra. If any of the lines were found to be rotated by the 
vapour, the direction of the rotation could be easily determined 
by rotating the analysing nicol until they were extinguished- 
