306 Prof. R. W. Wood on the Dispersion of Sodium 
A reticulated mesh was ruled on a small plate of glass 
with one of the Rowland dividing-engines. This plate was 
mounted in the eyepiece of the spectroscope on the slit of 
which the image of a horizontal slit was thrown, after dis- 
persion by the sodium tube. The spectroscope was provided 
with a plane grating ruled with 20,000 lines to the inch, 
which gave a very brilliant 2nd order spectrum. There were 
four squares of the reticulated mesh between the D lines, and 
nine squares between D, and Ds. 
As the temperature of the sodium tube was raised, the 
relative deviations in the different squares of the mesh were 
noted. The light immediately adjacent to the D lines was 
deviated entirely out of the field of the telescope before any 
measurable deviation at the helium line occurred. At a 
temperature at which the spectrum at the helium line was 
deviated the width of one square, the wave-lengths between 
the D lines, and for a distance of two squares to each side of 
them, were lost by absorption. By means of step by step 
measurements, taken at different temperatures, it was possible 
to get a very good idea of the dispersion curve within this very 
Fig. 3 
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narrow range of the spectrum. The appearance of the spectrum 
on the reticulated background at two different temperatures is 
shown in fig.3, the dispersion at D, being somewhat greater than 
at D,. Observations were taken of the dispersion within a 
quarter of a square of D,, that is within a distance of the 
absorption-line equal to 1/16 of the distance between the D 
lines. The deviation at this point was calculated to be about 
