he
interests of optics, not of chemistry--W. A. Miller,[374] and
Wheatstone. The last especially made a notable advance when, in the
course of his studies on the "prismatic decomposition" of the electric
light, he reached the significant conclusion that the rays visible in
its spectrum were different for each kind of metal employed as
"electrodes."[375] Thus indications of a wider principle were to be
found in several quarters, but no positive certainty on any single point
was obtained, until, in 1859, Gustav Kirchhoff, professor of physics in
the University of Heidelberg, and his colleague, the eminent chemist
Robert Bunsen, took the matter in hand. By them the general question as
to the necessary and invariable connection of certain rays in the
spectrum with certain kinds of matter, was first resolutely confronted,
and first definitely answered. It was answered affirmatively--else there
could have been no science of spectrum analysis--as the result of
experiments more numerous, more stringent, and more precise than had
previously been undertaken.[376] And the assurance of their conclusion
was rendered doubly sure by the discovery, through the peculiarities of
their light alone, of two new metals, named from the blue and red rays
by which they were respectively distinguished, "caesium," and
"rubidium."[377] Both were immediately afterwards actually obtained in
small quantities by evaporation of the Durckheim mineral waters.

The link connecting this important result with astronomy may now be
indicated. In the year 1802 it occurred to William Hyde Wollaston to
substitute for the round hole used by Newton and his successors for the
admittance of light to be examined with the prism, an elongated
"crevice" 1/20th of an inch in width. He thereupon perceived that the
spectrum, thus formed of light, as it were, _purified_ by the abolition
of overlapping images, was traversed by seven dark lines. These he took
to be natural boundaries of the various colours,[378] and satisfied with
this quasi-explanation, allowed the subject to drop. It was
independently taken up after twelve years by a man of higher genius. In
the course of experiments on light, directed towards the perfecting of
his achromatic lenses, Fraunhofer, by means of a slit and a telescope,
made the surprising discovery that the solar spectrum is crossed, not by
seven, but by thousands of obscure transverse streaks.[379] Of these he
counted some 600, and carefully ma