photography really owes its beginning to De la Rue, who used the
collodion process for the moon in 1853, and constructed the Kew
photoheliograph in 1857, from which date these instruments have been
multiplied, and have given us an accurate record of the sun's surface.
Gelatine dry plates were first used by Huggins in 1876.

It is noteworthy that from the outset De la Rue recognised the value
of stereoscopic vision, which is now known to be of supreme
accuracy. In 1853 he combined pairs of photographs of the moon in the
same phase, but under different conditions regarding libration,
showing the moon from slightly different points of view. These in the
stereoscope exhibited all the relief resulting from binocular vision,
and looked like a solid globe. In 1860 he used successive photographs
of the total solar eclipse stereoscopically, to prove that the red
prominences belong to the sun, and not to the moon. In 1861 he
similarly combined two photographs of a sun-spot, the perspective
effect showing the umbra like a floor at the bottom of a hollow
penumbra; and in one case the faculae were discovered to be sailing
over a spot apparently at some considerable height. These appearances
may be partly due to a proper motion; but, so far as it went, this was
a beautiful confirmation of Wilson's discovery. Hewlett, however, in
1894, after thirty years of work, showed that the spots are not always
depressions, being very subject to disturbance.

The Kew photographs [7] contributed a vast amount of information about
sun-spots, and they showed that the faculae generally follow the spots
in their rotation round the sun.

The constitution of the sun's photosphere, the layer which is the
principal light-source on the sun, has always been a subject of great