g any imperfections in the plate or in the image,
which may be remedied by the brush. In selecting brushes choose those
most susceptible of a fine point, which may be ascertained by wetting
them between the lips, or in a glass of water.

CHAP. XIII.

THE PHOTOGRAPHOMETER.

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 indi