steadfastly made its way, and for more than two centuries has been the
foundation of all the great discoveries in the stellar realm. Yet long
after the fact that the sun was the centre of the solar system was
well established no one understood why the planets should move in
their ceaseless, orderly procession around the central mass. To Newton
we owe the studies on the law of gravitation which brought us to our
present large conception as to the origin of this order. Starting with
the view that bodies attracted each other in proportion to their
weight, and in diminishing proportion as they are removed from each
other, Newton proceeded by most laborious studies to criticise this
view, and in the end definitely proved it by finding that the motions
of the moon about the earth, as well as the paths of the planets,
exactly agreed with the supposition.

The last great path-breaking discovery which has helped us in our
understanding of the stars was made by Fraunhofer and other
physicists, who showed us that substances when in a heated, gaseous,
or vaporous state produced, in a way which it is not easy to explain
in a work such as this, certain dark lines in the spectrum, or streak
of divided light which we may make by means of a glass prism, or, as
in the rainbow, by drops of water. Carefully studying these very
numerous lines, those naturalists found that they could with singular
accuracy determine what substances there were in the flame which gave
the light. So accurate is this determination that it has been made to
serve in certain arts where there is no better means of ascertaining
the conditions of a flaming substance except by the lines which its
light exhibits under this kind of analysis. Thus, in the manufacture
of iron by what is called the Bessemer process, it has been found very
convenient to judge as to the state of the molten metal by such an
analysis of the flame which comes forth from it.