t achievements of the nineteenth century, and again it is Bessel
to whom the honor of discovery is due. While testing his stars
for parallax; that astute observer was led to infer, from certain
unexplained aberrations of motion, that various stars, Sirius himself
among the number, are accompanied by invisible companions, and in
1840 he definitely predicated the existence of such "dark stars." The
correctness of the inference was shown twenty years later, when Alvan
Clark, Jr., the American optician, while testing a new lens, discovered
the companion of Sirius, which proved thus to be faintly luminous. Since
then the existence of other and quite invisible star companions has been
proved incontestably, not merely by renewed telescopic observations, but
by the curious testimony of the ubiquitous spectroscope.

One of the most surprising accomplishments of that instrument is the
power to record the flight of a luminous object directly in the line of
vision. If the luminous body approaches swiftly, its Fraunhofer lines
are shifted from their normal position towards the violet end of the
spectrum; if it recedes, the lines shift in the opposite direction. The
actual motion of stars whose distance is unknown may be measured in this
way. But in certain cases the light lines are seen to oscillate on the
spectrum at regular intervals. Obviously the star sending such light
is alternately approaching and receding, and the inference that it is
revolving about a companion is unavoidable. From this extraordinary test
the orbital distance, relative mass, and actual speed of revolution of
the absolutely invisible body may be determined. Thus the spectroscope,
which deals only with light, makes paradoxical excursions into the
realm of the invisible. What secrets may the stars hope to conceal when
questioned by an instrument of such necromantic power?

But the spectroscope is not alone in this audacious assault upon the
strongholds of nature. It has a worthy companion and assistant in
the photographic film, whose efficient aid has been invoked by the
astronomer even more recently. Pioneer work in celestial photography
was, indeed, done by Arago in France and by the elder Draper in America
in 1839, but the results then achieved were only tentative, and it was
not till forty years later that the method assumed really important
proportions. In 1880, Dr. Henry Draper, at Hastings-on-the-Hudson, made
the first successful photograph of a nebula.