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."