314 = Prof. R. W. Wood on the Dispersion of Sodium 
Absolute Determinations of the Refractive Index. 
The deviations of the light-rays by the non-homogeneous 
cylinders of sodium vapour formed in exhausted tubes gives 
us no measure of the absolute refractive index, for the reason 
that we have no way of determining the angle of the equiva- 
lent prism. Various methods were tried designed to give to a 
mass of vapour of uniform density a prismatic form. None 
of them was very satisfactory, and until the discovery was 
made that a fairly uniform cylinder of vapour could be formed 
in an exhausted tube, the absolute value of the index for any 
given wave-length could only be guessed at. 
The results found for the dispersion with the interfero- 
meter showed that results correct to within probably 5 per 
cent. could in all probability be obtained by this method. 
The density of sodium vapour at different temperatures has 
been investigated by Jewett (Phil. Mag. iv. p. 551, 1902) 
for a range comprised between 368° and 420°. It was ac- 
cordingly determined to take the temperature of the vapour 
as a measure of its density. 
A thermo-couple of iron and constantan wire was made by 
holding the ends of the wires together, both being connected 
to a storage-battery giving an H.M.F. of about 20 volts. 
The other wire of the battery was then touched momentarily 
to the tips of the two wires, the small are which formed on 
separation fusing them together very neatly. I do not re- 
member to have seen this method of making thermo-couples 
described, and have mentioned it as it was found very satis- 
factory. The fused bead which united the wires was then 
hammered out into a thin disk, and the junction mounted in 
the axis of the sodium tube, the wires being insulated with 
thin glass tubes which passed through a larger tube as shown 
in fig. 4. 
The couple was calibrated in baths of molten lead, zine, 
tin, and aluminium before and after the experiment. The 
determinations at the lower temperatures and densities were 
made with the light from the helium tube, those at high 
temperatures with the green light from a mercury tube. 
This was necessary since a mass of very dense sodium vapour 
not only absorbs helium light strongly, but owing to its high 
dispersion makes the fringes invisible, the different wave- 
lengths in the D line (which is of course of finite width) 
being shifted by different amounts. If the increase in the 
density with increasing temperature followed the same law as 
in the experiments of Jewett, the optically determined density- 
curve should agree with that found by the gravimetric method 
employed in his work. For example, he found that the 
