276 Prof. R. W. Wood on the Existence of 
No lines were found, however, which had just the right 
wave-length. It then occurred to uie that the selective 
rotatory power of the vapour could be utilized to furnish a 
source of light made up of just the right wave-lengths ; in 
other words, magnetized sodium vapour between crossed 
nicols could be used as a light filter. The light passed by 
the crossed nicols when the magnetic field was excited was 
accordingly sent through another magnetized tube of vapour 
and examined with a third nicol and spectroscope. It was 
hoped that by setting the third nicol for extinction, and 
causing the bright-line spectrum to appear again by excitation 
of the second magnet, it would be possible to determine the 
direction of rotation of the lines by observing in which 
direction it was necessary to rotate the third nicol in order 
to blot them out. The first magnet, with its sodium tube 
and polarizing prisms, delivers plane-polarized light of exactly 
the wave-lengths of the bright lines of the magnetic-rotation 
spectrum. This light is then passed through a second 
magnetized tube of sodium vapour, a nicol prism, and a 
spectroscope. The nicol having been set for extinction the 
bright-line spectrum disappeared, reappearing again as soon 
as the magnet was excited. It was found, however, that 
rotation of the third nicol was wholly without effect on the 
appearance of the lines, notwithstanding the fact that the 
light was originally plane-polarized. The magnetized sodium 
vapour appeared to have completely depolarized the light. 
The cause of this phenomenon is not difficult to explain. 
The lines which make up the magnetic-rotation spectrum, 
though they appeared as narrow as the iron arc-lines in a 
photograph which I made two years ago with a concave 
grating of 12 feet radius, are not in reality monochromatic. 
The action of an absorption-line is to rotate the plane of 
polarization of waves of nearly the same wave-length through 
various angles depending on their proximity to the absorption- 
line. It is these waves which are transmitted by the nicol. 
The line therefore has a finite, though narrow, width, and 
the second tube of magnetized vapour rotates the mono- 
chromatic constituents, of which the line is made up, through 
various angles. Some of the light in the line is therefore 
passed by the third nicol in every position. 
From their analogy to the bright rotated lines which 
border the D lines when examined under similar conditions, 
we should expect all of the lines of the magnetic-rotation 
spectrum to be double, and I have spent a good deal of time 
in attempts to show their duplicity, using an echelon grating. 
