h to exhibit such a satisfactory picture of the building
as had been hoped for. The outline of the roof and of the chimneys, &c.
against the sky was marked enough; but the details of the architecture
were feeble, and the parts in shade were left either blank or nearly so.
The sensitiveness of the paper to light, considerable as it seemed in some
respects, was therefore, as yet, evidently insufficient for the purpose of
obtaining pictures with the Camera Obscura; and the course of experiments
had to be again renewed in hopes of attaining to some more important
result.

The next interval of sufficient leisure which I found for the prosecution
of this inquiry, was during a residence at Geneva in the autumn of 1834.
The experiments of the previous spring were then repeated and varied in
many ways; and having been struck with a remark of Sir H. Davy's which I
had casually met with--that the _iodide_ of silver was more sensitive to
light than the _chloride,_ I resolved to make trial of the iodide. Great
was my surprise on making the experiment to find just the contrary of the
fact alleged, and to see that the iodide was not only less sensitive than
the chloride, but that it was not sensitive at all to light; indeed that
it was absolutely insensible to the strongest sunshine: retaining its
original tint (a pale straw colour) for any length of time unaltered in
the sun. This fact showed me how little dependance was to be placed on
the statements of chemical writers in regard to this particular subject,
and how necessary it was to trust to nothing but actual experiment: for
although there could be no doubt that Davy had observed what he described
under certain circumstances--yet it was clear also, that what he had
observed was some exception to the rule, and not the rule itself. In
fact, further inquiry showed me that Davy must have observed a sort of
subiodide in which the iodine was deficient as compared with the silver:
for, as in the case of the chloride and subchloride the former is much
less sensitive, so between the iodide and subiodide there is a similar
contrast, but it is a much more marked and complete one.

However, the fact now discovered, proved of immediate utility; for, the
iodide of silver being found to be insensible to light, and the chloride
being easily converted into the iodide by immersion in iodide of
potassium, it followed that a picture made with the chloride could be
_fixed_ by dipping it into a ba