th of the alkaline iodide.

This process of fixation was a simple one, and it was sometimes very
successful. The disadvantages to which it was liable did not manifest
themselves until a later period, and arose from a new and unexpected
cause, namely, that when a picture is so treated, although it is
permanently secured against the _darkening_ effect of the solar rays, yet
it is exposed to a contrary or _whitening_ effect from them; so that after
the lapse of some days the dark parts of the picture begin to fade, and
gradually the whole picture becomes obliterated, and is reduced to the
appearance of a uniform pale yellow sheet of paper.

A good many pictures, no doubt, escape this fate, but as they all seem
liable to it, the fixing process by iodine must be considered as not
sufficiently certain to be retained in use as a photographic process,
except when employed with several careful precautions which it would be
too long to speak of in this place.

During the brilliant summer of 1835 in England I made new attempts to
obtain pictures of buildings with the Camera Obscura; and having devised a
process which gave additional sensibility to the paper, viz. by giving it
repeated alternate washes of salt and silver, and using it in a moist
state, I succeeded in reducing the time necessary for obtaining an image
with the Camera Obscura on a bright day to ten minutes. But these
pictures, though very pretty, were very small, being quite miniatures.
Some were obtained of a larger size, but they required much patience, nor
did they seem so perfect as the smaller ones, for it was difficult to keep
the instrument steady for a great length of time pointing at the same
object, and the paper being used moist was often acted on irregularly.

During the three following years not much was added to previous knowledge.
Want of sufficient leisure for experiments was a great obstacle and
hindrance, and I almost resolved to publish some account of the Art in the
imperfect state in which it then was.

However curious the results which I had met with, yet I felt convinced
that much more important things must remain behind, and that the clue was
still wanting to this labyrinth of facts. But as there seemed no
immediate prospect of further success, I thought of drawing up a short
account of what had been done, and presenting it to the Royal Society.

However, at the close of the year 1838, I discovered a remarkable fact of
quite a new kind