The last number (for March, 1849) of the London Art-Journal, gives the
following description of a recent improvement in Photographic
Manipulation, and as I am desirous of furnishing everything new in the
art, I stop the press to add it, entire, to my work.
"Since the photographic power of the solar rays bears no direct
relation to their luminous influence, it becomes a question of
considerable importance to those who practice the beautiful art of
photography, to have the means of readily measuring the ever changing
activity of this force. Several plans more or less successful, have
been devised by Sir John Herschel, Messrs. Jordan, Shaw and Hunt. The
instrument, however, which is now brought forward by Mr. Claudet, who
is well known as one of our most successful Daguerreotypists, appears
admirably suited to all those purposes which the practical man
requires. The great difficulty which continually annoys the
photographic amateur and artist, is the determination of the
sensibility of each tablet employed, relatively to the amount of
radiation, luminous and chemical, with which he is working. With the
photographometer of Mr. Claudet this is easily ascertained. The
following woodcuts and concise description will sufficiently indicate
this useful and simple apparatus.
[Illustration: Fig. 30 (hipho_30.gif)]
"For an instrument of this kind it is important in the first place to
have a motion always uniform, without complicated or expensive
mechanism. This is obtained by means founded upon the principle of the
fall of bodies sliding down an inclined plane. The sensitive surface
is exposed to the light by the rapid and uniform passage of a metal
plate, A, B, (Fig. 31,) having openings of different length, which
follow a geometric progression. It is evident that the exposure to
light will be the same for each experiment, because the plate furnished
with the proportional openings falls always with the same rapidity, the
height of the fall being constant, and the angle of the inclined plane
the same. Each opening of this moveable plate allows the light to pass
during the same space of time, and the effect upon the sensitive
surface indicates 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. In this
opinion we are not singular; at a recent meeting of the Photographic
Club, to which this instrument was exhibited, it was with much real
satisfaction that we learned that several of our most eminent artists
were now eager and most successful students in Photography. The
beautiful productions of the more prominent members of this club
excited the admiration of all, particularly the copies of architectural
beauties, and small bits of landscape, by Messrs. Cundell and Owen. We
think that now the artist sees the advantage he may derive from the aid
of science, that both will gain by the union."
I hope the above description will induce our townsman, Mr. Roach, to
successfully produce an instrument that will meet the wants of our
artists in that part of the Daguerrean process referred to.