328 Prof. Wood on the Achromatization of Monochromatic 
the red side of the D lines, a similar drift of the position of 
maximum visibility was observed, and the direction of the 
drift was the same as before. In the case of helium light I 
have been able to increase the path difference to five or six 
centimetres, or to nearly treble it. 
The achromatizing action of the sodium vapour is most 
beautifully shown if we illuminate the interferometer with 
white light. 
Under ordinary conditions only two or three black and 
white fringes are seen, bordered on each side by perhaps a 
dozen rainbow-coloured bands, which fade rapidly into a 
uniform illumination. If sodium vapour is formed in one of 
the interferometer paths, the coloured fringes rapidly achro- 
matize, and increase in number, breaking up, however, into 
groups as shown in fig. 2. As the density of the vapour 
Fig, 2. 
I 
increases the number of groups increases, each group, how= 
ever, containing fewer fringes. The position of the centre of 
the grouped system drifts in the same direction as the point 
of maximum visibility in the previous experiments. 
The explanation of the altered appearance of the fringes in 
this case is not as simple as in those previously considered. 
We are dealing with two wide ranges of wave-lengths on 
opposite sides of the absorption-band. The fringe shifts of 
the two spectral regions will be in opposite directions, while 
the drifts of the points of maximum visibility will be in the 
same direction. It appeared as if this might increase the 
width of the region over which fringes could be observed, 
for the red-orange fringes are shifted in one direction and 
the yellow-green in the opposite. Hach set would be more 
or less perfectly achromatized, and in the region in which 
they overlapped we should expect a periodic visibility, owing 
to the difference in the widths of the fringes of the two 
systems. 
To test the point it seemed best to work with a narrow 
