The maximum of intensity is found at the edge of the most refrangible
violet rays, or a little beyond it, varying slightly according to the kind
of paper used, and the quantity of free nitrate of silver present.
The action commences at a point nearly coincident with the mean red
of the luminous spectrum, where it gives a dull ash or lead color,
while the most refrangible rays impress a ruddy snuff-brown,
the change of tint coming on rather suddenly about the end of
the blue or beginning of the violet rays of the luminous spectrum.
Beyond the extreme violet rays, the action rapidly diminishes,
but the darkening produced by these invisible rays, extends a
very small space beyond the point at which they cease to act on
the chloride of silver.

In its photographic application, it is, alone, of very little use;
but in combination with other reagents it becomes exquisitely sensitive.
With gallic acid and the ferrocyanate of potash it forms two of the most
sensitive photographic solutions with which we are acquainted.
These are used in the calotype process.

IODURET OF SILVER.--If upon a plate of polished silver we place
a small piece of iodine, and apply the heat of a lamp beneath
the plate for a moment, a system of rings is speedily formed.
The first ring, which spreading constantly forms the exterior
of the circle, is of a bright yellow color; within this,
there arises, sucessively, rings of green, red and blue colors,
and then again a fine yellow circle, centred by a greyish spot
on the place occupied by the iodine. On exposing these to the light,
the outer yellow circle almost instantly changes color,
the others slowly, in the order of their position, the interior
yellow circle resisting for a long time the solar influence.