cates exactly the intensity of the chemical rays. The
rapidity of the fall may be augmented or diminished by altering the
inclination of the plane by means of a graduated arc, C, D, (Fig. 30,)
furnished with a screw, E, by which it may be fixed at any angle. The
same result may be obtained by modifying the height of the fall or the
weight of the moveable plate. The photogenic surface, whether it be
the Daguerreotype plate, the Talbotype paper, or any other preparation
sensitive to light, is placed near the bottom of the inclined plane, F.
It is covered by a thin plate of metal, pierced with circular holes,
which correspond to the openings of the moveable plate at the moment of
the passage of the latter, during which the sensitive surface receives
the light wherever the circular holes leave it exposed.

[Illustration: Fig. 31 (hipho_31.gif)]

"The part of the apparatus which contains the sensitive surface is an
independent frame, and it slides from a dark box into an opening on the
side of the inclined plane.

"A covering of black cloth impermeable to light is, attached to the
sides of the moveable plate, enveloping the whole inclined plane,
rolling freely over two rollers, R, R, placed the one at the upper and
the other at the lower part of the inclined plane. This cloth prevents
the light striking the sensitive surface before and after the passage
of the moveable plate.

"It will be seen that this apparatus enables the experimentalist to
ascertain with great precision the exact length of time which is
required to produce a given amount of actinic change upon any sensitive
photographic surface, whether on metal or paper. Although at present
some calculation is necessary to determine the difference between the
time which is necessary for exposure in direct radiation, and to the
action of the secondary radiations of the camera obscura; this is,
however, a very simple matter, and it appears to us exceedingly easy to
adapt an instrument of this description to the camera itself.

"By this instrument Mr. Claudet has already determined many very
important points. Among others, he has proved that on the most
sensitive Daguerreotype plate an exposure of .0001 part of a second is
sufficient to produce a decided effect.

"Regarding photography as an auxiliary aid to the artist of no mean
value, we are pleased to record a description of an instrument which,
without being complicated, promises to be exceedingly useful.