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