ards such a mirror, stars appear immovably
fixed. The employment of long-focus lenses for coronal photography is
thus facilitated, and the size of the image is proportional to the
length of the focus. Professor Barnard, accordingly, depicted the
totality of 1900 with a horizontal telescope 61-1/2 feet long, fed by a
mirror 18 inches across, the diameter of the moon on his plates being 7
inches. The largest siderostat in the world is the Paris 50-inch
refractor, which formed the chief attraction of the Palais d'Optique at
the Exhibition of 1900. It has a focal length of nearly 200 feet, and
can be used either for photographic or for visual purposes.

Celestial photography has not reached its grand climacteric; yet its
earliest beginnings already seem centuries behind its present
performances. The details of its gradual yet rapid improvement are of
too technical a nature to find a place in these pages. Suffice it to say
that the "dry-plate" process, with which such wonderful results have
been obtained, appears to have been first made available by Sir William
Huggins in photographing the spectrum of Vega in 1876, and was then
successively adopted by Common, Draper, and Janssen. Nor should Captain
Abney's remarkable extension of the powers of the camera be left
unnoticed. He began his experiments on the chemical action of red and
infra-red rays in 1874, and at length succeeded in obtaining a
substance--the "blue" bromide of silver--highly sensitive to these
slower vibrations of light. With its aid he explored a vast, unknown,
and for ever invisible region of the solar spectrum, presenting to the
Royal Society, December 5, 1879,[1651] a detailed map of its infra-red
portion (wave-lengths 7,600 to 10,750), from which valuable inferences
may yet be derived as to the condition of the various kinds of matter
ignited in the solar atmosphere. Upon plates rendered "orthochromatic"
by staining with alizarine, or other dye-stuffs, the whole visible
spectrum can now be photographed; but those with their maximum of
sensitiveness near G are found preferable, except where the results of
light-analysis are sought to be completely recorded. And since
photographic refractors are corrected for the blue rays, exposures with
them of orthochromatic surfaces would be entirely futile.

The chemical plate has two advantages over the human retina:[1652]
First, it is sensitive to rays which are utterly powerless to produce
any visual effect; next,