300 = Prof. R. W. Wood on the Dispersion of Sodvum 
advantageous way to work was therefore to begin the 
observations with the maximum air path consistent with 
visibility of the fringes ; as the sodium vapour formed they 
increased in distinctness at first and then gradually faded 
away. In this way it was possible to get a shift twice as 
great as before ; if it was desirable to work with a denser 
vapour to eliminate the error due to evolved hydrogen, we 
had only to increase the length of the air path still further, 
and work further out in the system. Under these conditions 
the fringes are invisible at the beginning and end of the 
experiment, only coming into view when a considerable 
quantity of sodium vapour is present. The same thing 
is true to a less extent with helium light, the fringes dis- 
appearing after a shift of about 200 fringe-widths has taken 
place. By increasing the length of the air path, either by 
turning the screw of the instrument, or introducing one or 
more thick plates of plane-parallel glass, they again come 
into view, and the counting can be continued. The latter 
method was preferred, as the glass plate could be dropped 
into place in a moment without interrupting the count. This 
trick was only necessary in making the absolute determina- 
tions with helium light, as in the comparison work the vapour 
never attained such a density as to cause the helium fringes 
to disappear. When working close to the helium line with 
the monochromatic illuminator it was necessary to work with 
very narrow slits, otherwise the enormous dispersion of the 
vapour obliterated the fringes almost immediately, in spite 
of all precautions. It was impossible to approach closer to 
the absorption-band than the helium line, owing to this 
trouble, and ratios of 1:2 are about as large as we feel 
much confidence in, though on one or two occasions we set 
the monochromatic illuminator on the Ds line, and obtained 
equal shifts in the two systems. It was impossible to observe 
a shift of more than one or two fringes in this case before 
the system disappeared. The most accurate determinations 
are doubtless those where the ratio is between 1:3 and 1: 15. 
In the blue-green region the ratio is in the neighbourhood 
of 1:40, and the values are not as accurate, as an accidental 
displacement of half a fringe-width, which may easily occur 
as the result of a slight change in the temperature of the 
instrument or the air, exercises a much greater effect on the 
results. On this account the determinations made by the 
prismatic method are to be considered more reliable in this 
region of the spectrum. 
