been independently perfected by Professor Hale at Chicago, and
Deslandres at Paris.[8] They have succeeded in photographing the sun's
surface in monochromatic light, such as the light given off as one of
the bright lines of hydrogen or of calcium, by means of the
"Spectroheliograph." The spectroscope is placed with its slit in the
focus of an equatoreal telescope, pointed to the sun, so that the
circular image of the sun falls on the slit. At the other end of the
spectroscope is the photographic plate. Just in front of this plate
there is another slit parallel to the first, in the position where the
image of the first slit formed by the K line of calcium falls. Thus is
obtained a photograph of the section of the sun, made by the first
slit, only in K light. As the image of the sun passes over the first
slit the photographic plate is moved at the same rate and in the same
direction behind the second slit; and as successive sections of the
sun's image in the equatoreal enter the apparatus, so are these
sections successively thrown in their proper place on the photographic
plate, always in K light. By using a high dispersion the faculae which
give off K light can be correctly photographed, not only at the sun's
edge, but all over his surface. The actual mechanical method of
carrying out the observation is not quite so simple as what is here
described.

By choosing another line of the spectrum instead of calcium K--for
example, the hydrogen line H(3)--we obtain two photographs, one
showing the appearance of the calcium floculi, and the other of the
hydrogen floculi, on the same part of the solar surface; and nothing
is more astonishing than to note the total want of resemblance in the
forms shown on the two. This mode of research promises to afford many
new and useful data.