this may be done by the subject rubbing the face until it is very red.
The effect is to lessen the contrast, by giving the freckles and skin
the same color and the photogenic intensity of the red and yellow being
nearly the same, an impression can be produced perfectly clear.

When a child is to be taken, and there are doubts of its keeping still,
the operation may be accelerated by placing it nearer the window bringing
the screen nearer, and placing a white muslin cloth over the head;
this will enable you to work in one third of the usual time.
Should the person move, or the plate become exposed to the light,
it may be restored to its original sensitiveness by placing it over
the quick, one or two seconds.

Developing the Daguerreotype.--After the plate has been submittedto
the o peration of the light, the image is still invisible.
It requires to be exposed to the vapors of heated mercury.
It is not absolutely necessary to apply artificial heat to the mercury
to develop the image, for fair proofs have been produced by placing
a plate over the bath at the ordinary temperature of the atmosphere.
This plan, however, requires a long time and cannot be adopted
in practice, even if it were advisable. The time more usually
required in developing the image over the mercurial vapors,
is about two minutes, and the temperature is raised to a point
necessary to produce the desired effect in that time.
This point varies as indicated by different scales, but for
the ordinary scales it is not far from 90 deg. cen.

The mercury bath is accompanied with a centigrade thermometer,
by which the heat is regulated. Those furnished by the manufacturers
are not always correct, and it requires some experience to find