a less number into an inch. If a body giving only the number of
vibrations we call green were to approach sufficiently fast, it
would crowd in vibrations enough to appear what we call blue, indigo,
or even violet, according to its speed. If it receded sufficiently
fast, it would leave behind it only vibrations enough to fill up
[Page 53] the space with what we call yellow, orange, or red,
according to its speed; yet it would be green, and green only, all
the time. But how detect the change? If red waves are shortened they
become orange in color; and from below the red other rays, too far
apart to be seen by the eye, being shortened, become visible as red,
and we cannot know that anything has taken place. So, if a star
recedes fast enough, violet vibrations being lengthened become
indigo; and from above the violet other rays, too short to be seen,
become lengthened into visible violet, and we can detect no movement
of the colors. The dark lines of the spectrum are the cutting out of
rays of definite wave-lengths. If the color spectrum moves away,
they move with it, and away from their proper place in the ordinary
spectrum. If, then, we find them toward the red end, the star is
receding; if toward the violet end, it is approaching. Turn the
instrument on the centre of the sun. The dark lines take their
appropriate place, and are recognized on the ruled scale. Turn it on
one edge, that is approaching us one and a quarter miles a second by
the revolution of the sun on its axis, the spectral lines move
toward the violet end; turn the spectroscope toward the other edge
of the sun, it is receding from us one and a quarter miles a second
by reason of the axial revolution, and the spectral lines move
toward the red end. Turn it near the spots, and it reveals the
mighty up-rush in one place and the down-rush in another of one
hundred miles a second. We speak of it as an easy matter, but it is
a problem of the greatest delicacy, almost defying the mind of man