S. P. Langley—New method in Solar Spectrum Analysis. 145 
moved the other way, as if it were a scale which had been slid 
by a very slight but distinctly perceptible amount toward the 
ved end; while still the telluric lines retain their continuity, 
assuring us that no mal-adjustment has occurred. 
It will be admitted that this change is, if real, excellent ex- 
perimental evidence that the wave length is virtually different 
In light from the eastern and western limbs, as theory predicts. 
For, granting that the instrument is mal-adjusted in auy un- 
known way or degree, any instrumental cause will affect solar 
and telluric lines alike, and -we may in fact defy ingenuity to 
Suggest an error of adjustment, which will modify one and not 
the other. 
For the sake of clearness, I have assumed that we start with 
all the lines continuous in both spectra; in practice this condi- 
tion is not easily assured: commonly some lurking error, will, 
Without especial pains, cause them to appear broken upon a 
fixed source of light; but we disregard this, and consider, as 
we bring the instrament into new positions, only the difference 
of displacement of the solar and telluric lines. The simul- 
taneous observations of this difference, in each of two spectra, 
is the — condition relied on, not only in theory but in 
trum. . Of these we do know that they are either caused by 
the sun’s atmosphere or ours, without always knowing which, 
for these can only be inferred to be telluric from their growing 
to betray their solar origin. To merely see these two spectra 
With clearness, then, is to be enabled to pick out the telluric 
10pe then, it will not seem too assuming a title, if I speak 
of this as a new method in solar spectral analysis. 
T have only to add, that in all my trials of this method, I have 
3 a1 tm 
* Or by experiment on artificial lights view ig g 
