304 Prof. R. W. Wood on the Dispersion of Sodium 
trace of the periodic invisibility was found in the fringe 
system, such as is always observed when ordinary sodium 
light is used. As soon as the smallest trace of sodium 
vapour was formed in the tube, the fringes were found to 
have become quite invisible in certain parts of the system. 
The visibility curve changed rapidly as the density of the 
vapour increased, the period becoming smaller. The general 
behaviour of the fringes did not differ materially from that 
observed when both sodium lines were used, proving that the 
presence of the two wave-lengths was not essential. The 
appearance of fringes under the conditions of the experiment 
depends upon the fact that the emission-line is broader than 
the absorption-line. The absorption is restricted to the centre 
of the line, the edge light only getting through the sodium 
tube and forming fringes. This edge light is most power- 
fully dispersed by the vapour, and the two fringe systems 
which it forms are shifted in opposite directions. It was 
hoped that a method might be worked out for determining 
the dispersion of the vapour in the immediate vicinity of the 
D lines, by studying the behaviour of sodium light when 
dispersed in the interferometer, and a great deal of time was 
lost in the endeavour to unravel the very complicated changes 
produced in the appearance of the fringe system. If we 
were dealing with strictly homogeneous radiations, lying to 
the right and left respectively of the absorption-band, the 
two fringe systems would move in opposite directions at the 
same rate, and we should have simply rapid alternations of 
visibility and invisibility. By the simultaneous use of a 
helium tube we could determine the number of disappearances 
of the sodium fringes corresponding to a shift of one or more 
helium fringes, which would give us at once a measure of 
the shift of the system formed by the sodium light. This 
would give us a measure of the refractive index closer to the 
absorption-bands than we could get by any other method. 
An attempt was made to utilize the Zeeman phenomenon in this 
case, the sodium flame being placed in a magnetic field and 
the interferometer illuminated with one or the other of the D 
lines, by means of the polarizing system. There was no 
difficulty in finding positions of the interferometer mirror 
for which the fringes were visible with the field on, and 
invisible when it was off; but nothing resembling a rapid 
appearance and disappearance of the fringes was observed 
when the sodium vapour was formed. This is probably due 
to a number of causes. In the first place, the light from the 
flame is by no means strictly homogeneous, for a pale flame, 
containing only a little sodium and yielding narrow lines, 
