57,
when the want was supplied. De la Rue's new observatory, built in that
year at Cranford, was expressly dedicated to celestial photography; and
there he applied to the heavenly bodies the stereoscopic method of
obtaining relief, and turned his attention to the delicate business of
photographing the sun.

A solar daguerreotype was taken at Paris, April 2, 1845,[447] by
Foucault and Fizeau, acting on a suggestion from Arago. But the attempt,
though far from being unsuccessful, does not, at that time, seem to have
been repeated. Its great difficulty consisted in the enormous
light-power of the object to be represented, rendering an inconceivably
short period of exposure indispensable, under pain of getting completely
"burnt-up" plates. In 1857 De la Rue was commissioned by the Royal
Society to construct an instrument specially adapted to the purpose for
the Kew Observatory. The resulting "photoheliograph" may be described as
a small telescope (of 3-1/2 inches aperture and 50 focus), with a
plate-holder at the eye-end, guarded in front by a spring-slide, the
rapid movement of which across the field of view secured for the
sensitive plate a virtually instantaneous exposure. By its means the
first solar light-pictures of real value were taken, and the autographic
record of the solar condition recommended by Sir John Herschel was
commenced and continued at Kew during fourteen years--1858-72. The work
of photographing the sun is now carried on in every quarter of the
globe, from Mauritius to Massachusetts, and the days are few indeed on
which the self-betrayal of the camera can be evaded by our chief
luminary. In the year 1883 the incorporation of Indian with Greenwich
pictures afforded a record of the state of the solar surface on 340
days; and 364 were similarly provided for in 1897 and 1899.

The conclusions arrived at by photographic means at Kew were
communicated to the Royal Society in a series of papers drawn up jointly
by De la Rue, Balfour Stewart, and Benjamin Loewy, in 1865 and
subsequent years. They influenced materially the progress of thought on
the subject they were concerned with.

By its rotation the sun itself offers opportunities for bringing the
stereoscope to bear upon it. Two pictures, taken at an interval of
twenty-six minutes, show just the amount of difference needed to give,
by their combination, the maximum effect of solidity.[448] De la Rue
thus obtained, in 1861, a stereoscopic view of a sun-s