302 Prof. R. W. Wood on the Dispersion of Sodium 
of phases of maximum visibility, the method being the one 
commonly employed in the laboratory in the determination 
of the relative wave-lengths of the sodium lines. The mag- 
netic field was now thrown off, and the temperature of the 
heating coil gradually raised. As the sodium vapour formed, 
the fringes were counted as they drifted across the field. 
The current was thrown into the magnet at frequent intervals 
for a fraction of a second, and the fringes were seen to pass 
through successive phases of visibility and invisibility. The 
momentary disappearance caused by the magnetic field did 
not interfere with the counting, for the fringes moved slowly 
and with the regularity of clockwork. The exact moment 
at which the fringes disappeared could not of course be 
accurately determined, since they remained invisible during 
a period occupied by a shifting of several fringe-widths, but 
by observing six or seven successive disappearances values 
were obtained which did not differ by more than two or three 
per cent. The average of a number of readings showed that 
the sodium vapour shifted the two sets of fringes in the ratio 
of 10°5 to 11, that is fringes formed by waves of length 
587487 are shifted 10°5 fringe-widths, while those formed by 
waves of length 587513 are shifted 11 fringe-widths. These 
numbers do not look very formidable until we translate them 
into prismatic deviations. A prism having the same disper- 
sion giving a total deviation of only 11 degrees would 
separate the two components of the Zeeman triplet half a 
degree, or if we could construct a sodium-vapour prism giving 
the same deviation as a 60° glass prism, two lines in the 
spectrum twenty-three times as close together as the D lines 
would appear separated by a distance greater than the distance 
between the red and the bluish green of the spectrum formed 
by the glass prism. 
The same method was applied to a determination of the 
dispersion between the yellow mercury lines, for the purpose 
of obtaining a check on the curve obtained by comparing the 
shifts of the helium fringes with fringes formed by the light 
from the monochromatic illuminator. 
A prism was placed in front of the observing telescope to 
separate the yellow fringes from the more intense green ones. 
On setting them in motion by forming sodium vapour in the 
path of the light, they were found to disappear periodically in 
the same manner. It was impossible to get as accurate data 
as were obtained with the Zeeman lines, since there were 
moments of complete invisibility, but by counting steadily 
it was possible to tide over the few seconds when the field 
was a blank and get a fair estimate of the relative shifts. 
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