312 Prof. R. W. Wood on the Dispersion of Sodium 
internal diameter, was fitted with end-plates of quartz, loaded 
with sodium and exhausted. The arrangement of the appa- 
ratus is shown in fig. 5. As the method 
depends on forming a sharp image of a 
brightly illuminated horizontal slit upon 
the vertical slit of the spectrograph, me 
quartz lenses could not be used owing to ne = 
their chromatic aberration. | Concave ee h 
silvered mirrors are free from this defect, ry ' 
but have abnormally low reflecting power il i 
in the ultra-violet region. Very satis- " i 
factory photographs were obtained with : i 
them, however, in the preliminary work, t" 
though they were replaced by magna- 
lium reflectors later on. The light from von 
the crater of an arc-lamp was focussed on S . 
the horizontal slit, and collimated with ae 
one of the concave mirrors. It then 
passed through the sodium tube, after A 
which it was brought to a focus by a 
second mirror on the slit of a quartz spec- 
trograph. The tube was heated by a row of small Bunsen 
burners, and as soon as a steady state was reached, which could 
be told by observing the curved branches of the spectrum with 
an eyepiece, the photographic plate was inserted and exposed. 
Owing to the low reflecting power of the silver the ultra- 
violet. region was somewhat underexposed. The curvature 
of the spectrum in the immediate vicinity of the first two 
ultra-violet lines of the principal series was very pronounced, 
however. ‘The plates obtained with the small quartz spectro- 
graph are too small to reproduce well, especially as the 
density is not very ereat in the most interesting region, and 
++ has seemed better on this account to make drawings from 
them. The general appearance of the photographed spectrum 
is shown in fig. 2 (p. 296), which has been enlarged about two 
diameters. It is apparent that the effect of these lines on the 
refractivity of the medium is negligible except in their 
immediate vicinity. 
Plates obtained in the manner described were useless for 
purposes of measurement, as no record was made of the 
position of the spectrum on the plate before its deviation by 
the sodium prism. 
As the displacement is very slight in the remote ultra- 
violet the two records would overlap in this region, even if 
the spectrum was very narrow. To overcome this difficulty 
the following method was devised. A slide was fitted to the 
Fig. 5. 
: 
