ty of preventing the feeble light from the prominence from being
overpowered by extraneous light, the photography of these bodies was not
very successful until Professor Hale, of Chicago, designed his
spectro-heliograph. In this instrument there is (in addition to the
usual slit through which the light falls on the prisms, or grating,) a
second slit immediately in front of the photographic plate through which
the light of a given wave-length can be permitted to pass to the
exclusion of all the rest. The light chosen for producing an image of
the prominences is that radiated in the remarkable "K line," due to
calcium. This lies at the extreme end of the violet. The light from that
part of the spectrum, though it is invisible to the eye, is much more
active photographically than the light from the red, yellow, or green
parts of the spectrum. The front slit is adjusted so that the K line
falls upon the second slit, and as the front slit is slowly swept by
clockwork over the whole of a prominence, the second slit keeps pace
with it by a mechanical contrivance.

If the image of the solar disc is hidden by a screen of exactly the
proper size, the slits may be made to sweep over the whole sun, thus
giving us at one exposure a picture of the chromospheric ring round the
sun's limb with its prominences. The screen may now be withdrawn, and
the slits may be made to sweep rapidly over the disc itself. They reveal
the existence of glowing calcium vapours in many parts of the surface of
the sun. Thus we get a striking picture of the sun as drawn by this
particular light. In this manner Professor Hale confirmed the
observation made long before by Professor Young, that the spectra of
faculae always show the two great calcium bands.

The velocity with which a prominence shoots upward from the sun's limb
can, of course, be measured directly by observations of the ordinary
kind with a micrometer. The spectroscope, however, enables us to
estimate the speed with which disturbances at the surface of the sun
travel in the direction towards the earth or from the earth. We can
measure this speed by watching the peculiar behaviour of the spectral
lines representing the rapidly moving masses. This opens up a remarkable
line of investigation with important applications in many branches of
astronomy.

It is, of course, now generally understood that the sensation of light
is caused by waves or undulations which impinge on the retina of the
eye a