ed and
measured by Fraunhofer with such masterly skill, that they are now
universally known as Fraunhofer's lines. To give an explanation of
these lines was, as I have said, a problem which long challenged the
attention of philosophers, and to Professor Kirchhoff belongs the
honour of having first conquered this problem.

(The positions of the principal lines, lettered according to
Fraunhofer, are shown in the annexed sketch (fig. 55) of the solar
spectrum. A is supposed to stand near the extreme red, and J near the
extreme violet.)

[Illustration: Fig. 55.]

The brief memoir of two pages, in which this immortal discovery is
recorded, was communicated to the Berlin Academy on October 27, 1859.
Fraunhofer had remarked in the spectrum of a candle flame two bright
lines, which coincide accurately, as to position, with the double dark
line D of the solar spectrum. These bright lines are produced with
particular intensity by the yellow flame derived from a mixture of
salt and alcohol. They are in fact the lines of sodium vapour.
Kirchhoff produced a spectrum by permitting the sunlight to enter his
telescope by a slit and prism, and in front of the slit he placed the
yellow sodium flame. As long as the spectrum remained feeble, there
always appeared two bright lines, derived from the flame, in the place
of the two dark lines D of the spectrum. In this case, such absorption
as the flame exerted upon the sunlight was more than atoned for by the
radiation from the flame. When, however, the solar spectrum was
rendered sufficiently intense, the bright bands vanished, and the two
dark Fraunhofer lines appeared with much greater sharpness and
distinctness than when the flame was not employed.

This result, be it noted, was not due to any real quenching of the
bright lines of the flame, but to the augmentation of the intensity of
the adjacent spectrum. The experiment proved to demonstration, that
when the white light sent through the flame was sufficiently intense,
the quantity which the flame absorbed was far in excess of that which
it radiated.

Here then is a result of the utmost significance. Kirchhoff
immediately inferred from it that the salt flame, which could
intensify so remarkably the dark lines of Fraunhofer, ought also to be
able to _produce_ them. The spectrum of the Drummond light is known to
exhibit the two bright lines of sodium, which, however, gradually
disappear as the modicum of sodium, contained as an impu