320 Prof. R. W. Wood on the Dispersion of Sodium 
The constant m in the above formula can be determined 
from a single observation of the refractivity. It was deter- 
mined from two values, namely, the refractivity for the green 
line of mercury (A=546) and that for A=585, the latter wave- 
length being quite close to the D lines. Since the refractivity 
has widely different values for these two wave-lengths, we 
should expect the difference between the two calculated values 
to be a maximum in this case, in the event of the dispersion 
being incorrectly represented by the formula. The values 
for m found in the two cases were 0°000056 and 0°000054, a 
surprisingly close agreement. The mean value m=0:00055 
was taken, and the indices for a number of wave-lengths 
calculated. Some of these values are given in the table of 
refractive indices, and they will be found to agree very 
closely with the observed values. ‘The values calculated 
between the helium line and the D lines are given in the 
second part of the table. | 
It will be noticed that the greatest discrepancies are found 
in the ultra-violet, the observed values indicating a greater 
deviation (negative since the index is less than unity) than 
we should expect theoretically. In other words, we should 
expect the deviated spectrum to be more nearly in coinci- 
dence with the undeviated than was found in the photographs 
taken through the slotted slide. I am inclined to attribute 
the discrepancy to the presence of hydrogen in the tube. 
The dispersion-tube was exhausted and sealed off from the 
pump before introduction into the combustion-furnace. A 
small amount of hydrogen is always liberated from the sodium, 
as scon as it is heated, and without doubt the tube contained | 
a small amount of this gas during the experiment. At the 
time it was not expected that this would give any trouble, 
but observations appear to indicate the contrary. <A tube 
containing air or any ordinary gas acts like a prism with its 
thin edge down, when the floor of the tube is strongly heated. 
This is owing to the rarefaction of the gas along the heated 
floor. Now the sodium prism is turned in exactly the oppo- 
site way, the denser portions being close to the floor. But 
on the blue side of the sodium absorption-band the index of 
refraction is less than unity; consequently the oppositely 
oriented prisms (hydrogen and sodium vapour) deviate in 
the same direction, and the effect of the hydrogen prism will 
be greatest in the ultra-violet for obvious reasons. The 
result will be that slightly too great deviations will be found 
in this region, which is precisely what we require to explain 
the discrepancies between the observed and the calculated 
values. 
