1905-6.] Dr J. Halm on the Spectro-heliometer. 
77 
in the diagram fig. 1, where the upper spectrum represents the 
“ receding ” limb, i.e. that limb at which the solar gases, owing to 
rotation, move away from the observer, while the lower spectrum 
shows the “approaching” limb. The diagram indicates the 
positions of the absorption lines employed in the measurements. 
Two of these, b and d , are of solar origin, i.e. they belong to a 
vapour (iron) present in the sun’s reversing layer, while the two 
others, a and c, are telluric lines caused by the absorption of the 
Spectrum 
of 
receding 
limb. 
Solar 
limbs. 
Spectrum 
of 
approach- 
ing limb. 
Fig. 1. — Group of lines as seen in the viewing telescope. 
(a and c are telluric, b and d solar lines. ) 
cold oxygen gas of our own atmosphere. They are members of 
the well-known a-group of the solar spectum. Obviously the lines 
b and d are affected by solar rotation, their displacements due to 
this cause being indicated in the diagram, whereas the telluric 
lines a and c suffer no such shift. 
The observation consists in measuring, by means of a filar 
micrometer, in both spectra the distances ab and cd , which we 
may conveniently call [a&] R and [cd] R for the receding limb, and 
[a&] A and [cd] A for the approaching limb. Then the differences 
[a&] R — [a&] A and [c^] R — \cd\ represent the double displacements 
of each solar line due to rotation of the sun. It is easily seen 
