Before taking leave of the subject of photogenic drawing, I must
mention one or two facts, which may be of essential service to
operators.
It has been observed by Daguerre, and others, in Europe, and probably
by some of our own artists, that the sun two hours after it has passed
the meridian, is much less effective in the photographic process, than
it is two hours previous to its having reached that point. This may
depend upon an absorptive power of the air, which may reasonably be
supposed to be more charged with vapor two hours before noon. The fuse
of the hygrometer may possibly establish the truth or falsity of this
supposition. The fact, however, of a better result being produced
before noon being established, persons wishing their portraits taken,
will see the advantage of obtaining an early sitting, if they wish good
pictures. On the other hand, if the supposition above mentioned prove
true, a too early sitting must be avoided.
If we take a considerable thickness of a dense purple fluid, as, for
instance, a solution of the ammonia-sulphate of copper, we shall find
that the quantity of light is considerably diminished, at least
four-fifths of the luminous rays being absorbed, while the chemical
rays permeate it with the greatest facility, and sensitive preparations
are affected by its influence, notwithstanding the deficiency of light,
nearly as powerfully as if exposed to the undecomposed sunbeams.
It was first imagined that under the brilliant sun and clear skies of
the south, photographic pictures would be produced with much greater
quickness than they could be in the atmosphere of Paris. It is found,
however, that a much longer time is required. Even in the clear and
beautiful light of the higher Alps, it has been proved that the
production of the photographic picture requires many minutes more, even
with the most sensitive preparations, than it does in London. It has
also been found that under the brilliant light of Mexico, twenty
minutes, and half an hour, are required to produce effects which in
England would occupy but a minute; and travellers engaged in copying
the antiquities of Yucatan have on several occasions abandoned the use
of the photographic camera, and taken to their sketch books. Dr.
Draper* has observed a similar difference between the chemical action
of light in New York and Virginia. This can be only explained by the
supposition that the intensity of the light and heat of these climes
interferes with the action of the ENERGIC rays on those sensitive
preparations which are employed.
* I would here take occasion to remark that our country man, Dr.
Draper, is very frequently quoted by Mr. Hunt in his "Researches."