spirit lamp, it gives it a yellow colour, and its spectrum is a bright
yellow line agreeing in position with line D of the solar spectrum.

In 1832 Sir David Brewster found some of the solar black lines
increased in strength towards sunset, and attributed them to
absorption in the earth's atmosphere. He suggested that the others
were due to absorption in the sun's atmosphere. Thereupon Professor
J. D. Forbes pointed out that during a nearly total eclipse the lines
ought to be strengthened in the same way; as that part of the sun's
light, coming from its edge, passes through a great distance in the
sun's atmosphere. He tried this with the annular eclipse of 1836,
with a negative result which has never been accounted for, and which
seemed to condemn Brewster's view.

In 1859 Kirchoff, on repeating Frauenhofer's experiment, found that,
if a spirit lamp with salt in the flame were placed in the path of the
light, the black D line is intensified. He also found that, if he used
a limelight instead of the sunlight and passed it through the flame
with salt, the spectrum showed the D line black; or the vapour of
sodium absorbs the same light that it radiates. This proved to him the
existence of sodium in the sun's atmosphere.[4] Iron, calcium, and
other elements were soon detected in the same way.

Extensive laboratory researches (still incomplete) have been carried
out to catalogue (according to their wave-length on the undulatory
theory of light) all the lines of each chemical element, under all
conditions of temperature and pressure. At the same time, all the
lines have been catalogued in the light of the sun and the brighter of
the stars.