Interference Fringes by a Highly Dispersive Medium. 825 
which it is possible to obtain interference-fringes with helium 
(D;) light can be more than doubled by the introduction of 
a small amount of sodium vapour into the path of one of the 
interfering beams. This development of fringes far out in 
the system by the dispersive action of the vapour is accom- 
panied by their complete disappearance at the centre of the 
system, where the difference of path is zero. 
In order to understand this action of the vapour we must 
first consider briefly the conditions under which fringes may 
be visible. 
Suppose that we have a system of circular fringes formed 
with white light, and consider a point just outside of the 
visible ring system, where the illumination appears uniform. 
Our fringe system is built up of an infinite number of coloured 
systems which are in coincidence at the centre, but which 
get more and more out of step as we advance out into the 
system, owing to the fact that the “scale” on which the 
Newton rings are formed decreases with decreasing wave- 
length. Let us now consider in what manner fringes may 
be made to appear at a point where the overlapping is so 
great as to destroy all trace of the fringes; in other words, 
how may achromatization be more or less completely secured. 
It appears to me that there are only two conceivable ways 
in which the result can be obtained. If we could, by the 
introduction of a dispersing medium, increase the diameters 
of the blue rings without greatly affecting the diameters of 
the red ones, it is obvious that we should greatly increase the 
number of visible fringes without, however, altering their 
distinctness at the centre of the system. 
A slight inclination of either of the back mirrors of the 
interferometer increases or diminishes the scale on which 
the fringes are formed, and since a similar change in the 
direction of the reflected rays can he effected by the intro- 
duction of an acute prism, it is easy to see that, owing to the 
dispersion of the latter, the change in the scale will be 
different for the different wave-lengths, more or less perfect 
achromatization resulting. 
The introduction of a medium into the path of one of the 
interfering beams causes a shift of the fringe system as a 
whole, and if the medium is dispersing the shifts will be 
different for the different colours. The red, green, and blue 
fringes, which are out of step at a given point, may thus be 
brought into coincidence by the inequality of their respective 
displacements. In this case, however, since the systems are 
shifted as a whole, the fringes will be thrown out-of-step at 
the centre of the system, consequently we have obtained an 
Phil. Mag. 8. 6. Vol. 8. No. 45. Sept. 1904. Z 
