Interference Fringes by a Highly Dispersive Medium. 329 
range of the spectrum symmetrical about the D_ lines. 
This was obtained by opening the slit of the monochromatic 
illuminator, bisecting it with a wire, and adjusting the prisms 
so that the region of the D lines was screened off by the 
wire. By means of a small screen either of the two narrow 
portions of the spectrum bordering the D lines could be 
screened off. 
The effect of the sodium vapour on the fringes formed 
when the interferometer was illuminated by either one or 
both of the two portions of the spectrum could then be studied 
at leisure. 
It was found that when a considerable amount of the 
vapour was present the apparent centre of the greenish- 
yellow fringe system was widely separated from the centre 
of the orange-yellow system. 
When both sorts of light were used at once there was a 
periodic visibility in the region in which the two systems 
overlapped, the appearance in the three cases being shown 
) 
In fig. 3. 
The case is a little more complicated when white light, 
or the entire spectrum, is used, but it does not differ materially 
from the special case just considered. 
Practically the same thing occurs when the interferometer 
is illuminated with sodium light, except that in this case 
the density of the sodium vapour in the optical path must 
be very much smaller. A periodic visibility results even 
when the light of one of the D lines is removed by the 
polarizing system described in the previous paper. The 
case is of course similar to the last-mentioned, for the width 
of the D line illuminating the instrument is greater than 
the width of the absorption-band of the rare vapour. We 
thus have a condition identical with that which we had when 
