1901-2.] Professor C. Piazzi Smyth on Sodium lines . 
225 
Does the Spectrum-place of the Sodium lines vary in 
different Azimuths? By the late Professor C. Piazzi 
Smyth, Astronomer-Royal for Scotland. Communicated by 
Professor J. G. MacGregor, D.Sc., F.R.S. 
(Read May 5, 1902.) 
The above question having been set before me by my friend 
Prof. P. G. Tait, and in such a guise that its practical solution, if 
amounting to anything sensible, might have astronomical applica- 
tions, I set myself to examine it with the highest dispersion power 
in my possession, viz., a fine Rutherfurd Diffraction grating, of 
17,296 lines to the inch, ruled over a surface P6 inch square; a 
telescope 52 inches long, with magnifying powers from 20 to 50 ; 
and a collimator 32 inches long, armed with a very substantial 
slit-apparatus by Mr Adam Hilger, and some other fittings. 
These were all laid out in horizontal plane on the levelled top 
of a table, which revolved on three wheels below, in a circle 
divided to every ten degrees of astronomically determined azimuth, 
on the floor of an upper chamber. 
Some preliminary trials were made with sodium light, de- 
rived from salt burning in a Bunsen-burner gas flame; first, by 
placing burners on either side of the slit, and sending their lights 
into that by metal reflectors placed oppositely to each other and 
at 45° each to the axial direction. This plan, therefore, gave two 
images of the salt-lines (say D 1 and D 2 ), one above the other in 
the field of view. The second plan consisted in sending the light 
of a single burner direct into the slit, and noting by micrometer the 
absolute spectrum place of one of its D lines, while the table was 
turned to successive steps of azimuth all round the circle. 
No change of spectrum place, as depending on azimuth, could 
be established by either of these methods ; but the images of the 
lines were so barbarously coarse and hazy, that it was hazardous to 
attempt to say from them within what fraction of the distance 
from D 1 to D 2 the negative could be considered absolutely proved. 
I therefore arranged a variety of the apparatus for trying the 
PROC. ROY. SOC. EDIK — VOL. XXIV. 15 
