Vapour in the Visible and Ultra-violet Regions. 301 
Determination of the Dispersion in the immediate vicinity 
of the D Lines. 
The extraordinarily high value of the dispersion close to the 
absorption-band makes it impossible to sift out from the 
spectrum, light sufficiently homogeneous to yield interference- 
fringes when even a very small amount of sodium vapour is 
introduced into the path of the light. It was found difficult 
to get satisfactory values with the light from the monochro- 
matic illuminator much closer to D; than the distance 
separating D; from D,, the fringes becoming blurred as soon 
as the smallest trace of sodium vapour was formed. A differ- 
ent method was accordingly adopted which yielded excellent 
results. 
It is clear that if we employ light of two different wave- 
lengths in illuminating the interferometer, the introduction 
of the metallic vapour will shift the two fringe systems by 
different amounts, the fringes appearing and disappearing as 
they get into and out of step in succession. If both systems 
are shifted in the same direction, which is the case when both 
waye-lengths are on the same side of the absorption-band, 
we can measure the dispersion by counting the number of 
fringes which pass the cross-hair of the telescope between two 
successive disappearances. If, for example, the number is 
found to be 15, we know that one set of waves is retarded 
or accelerated 15 wave-lengths, under the same conditions 
which cause a shift of 16 fringes for the other waves. All 
uncertainty as to the movement of the fringes during a period 
of minimum visibility was avoided in the method employed, 
as will appear presently. 
To determine the dispersion close to the D lines we require 
highly homogeneous light of two different wave-lengths, 
which would yield a close double line at the point at which 
we wish to measure the dispersion. 
This was accomplished by breaking up the radiation of a 
helium tube into a Zeeman triplet and cutting out the middle 
component by means of a Nicol prism placed with its plane 
of vibration perpendicular to the lines of magnetic force. 
A large Ruhmkorff magnet with an ammeter in circuit was 
employed, the key for making and breaking the circuit being 
placed close to the observing telescope of the interferometer. 
The distance between the components of the double line 
obtained when a current of 30 amperes was flowing through 
the magnet was found to be 1/23 of the distance between 
the sodium lines. This was determined very easily by 
counting the number of turns of the interferometer-screw 
necessary to cause the fringes to pass through a given number 
