solid shell of the planet, and if they were of sufficient distinctness
to be seen in substantially the same form by all observers armed with
competent instruments, then whatever conclusion was drawn from their
apparent motion as to the period of the planet's rotation would have to
be accepted. In the case of Mercury the markings, while not easily seen,
appear to be sufficiently distinct to afford confidence in the result of
observations based upon them, but Venus's markings have been represented
in so many different ways that it seems advisable to await more light
before accepting any extraordinary, and in itself improbable, conclusion
based upon them.

It should also be added that in 1900 spectroscopic observations by
Belopolski at Pulkova gave evidence that Venus really rotates rapidly on
her axis, in a period probably approximating to the twenty-four hours of
the earth's rotation, thus corroborating the older conclusions.

Belopolski's observation, it may be remarked, was based upon what is
known as the Doppler principle, which is employed in measuring the
motion of stars in the line of sight, and in other cases of rapidly
moving sources of light. According to this principle, when a source of
light, either original or reflected, is approaching the observer, the
characteristic lines in its spectrum are shifted toward the blue end,
and when it is retreating from the observer the lines are shifted toward
the red end. Now, in the case of a planet rotating rapidly on its axis,
it is clear that if the observer is situated in, or nearly in, the plane
of the planet's equator, one edge of its disk will be approaching his
eye while the opposite edge is retreating, and the lines in the spectrum
of a beam of light from the advancing edge will be shifted toward the
blue, while those in the spectrum of the light coming from the
retreating edge will be shifted toward the red. And, by carefully noting