green. Over the space
occupied by the extreme red rays, a protecting influence is again
exerted; the space is retained free from mercurial vapor and the band
is found to surround the whole of the least refrangible rays, and to
unite itself with the band which surrounds the rays of greatest
refrangibility. This band is not equally well defined throughout its
whole extent. It is most evident from the extreme red to the green; it
fades in passing through the blue, and increases again, as it leaves
the indigo, until beyond the invisible chemical rays it is nearly as
strong as it is at the calorific end of the spectrum.

Images on Daguerreotype plates which have been completely obliterated
by rubbing may be restored, by placing it in a tolerably strong
solution of iodine in water.

BROMIDE OF SILVER.--This salt, like the iodide, does not appear to be
readily changed by the action of light; but when combined with the
nitrate of silver it forms a very sensitive photographic preparation.

Paper prepared with this salt, blackens over its whole extent with
nearly equal intensity, when submitted to the prismatic spectrum. The
most characteristic peculiarity of the spectrum is its extravagant
length. Instead of terminating at the mean yellow ray, the darkened
portion extends down to the very extremity of the visible red rays. In
tint it is pretty uniformly of a grey-black over its whole extent,
except that a slight fringe of redness is perceptible at the least
refracted end. Beyond the red ray, an extended space is protected from
the agency of the dispersed light, and its whiteness maintained; thus
confirming the evidence of some chemical power in action, over a space
beyond the luminous spectrum, which corresponds with the rays of the
least refrangibility.

This salt is extensively used in photographic drawing.

PREPARATIONS OF GOLD.--Chloride of Gold, freed from an excess of acid
is slowly changed under the action of light; a regularly increasing
darkness taking place until it becomes purple, the first action of the
light being to whiten the paper, which, if removed from the light at
this stage, will gradually darken and eventually develope the picture.
This process may be quickened by placing the paper in cold water.

Chloride of gold with nitrate of silver gives a precipitate of a yellow
brown color. Paper impregnated with the acetate of lead, when washed
with perfectly neutral chloride of gold, acquires a browni