456 PROFESSOR LUDWIG MOSER ON VISION, 
and that these plates may be again exposed to the vapours of 
iodine without diminishing their sensibility. 
By these experiments I think I have proved that contact, — 
condensation of vapours, and light produce the same effect on all 
bodies. The differences which appear may be referred to the — 
varying intensity of the producing cause, and the greater or less 
depth to which the action extends. The black iodide of silver, , 
for instance, is obtained with more difficulty than is generally 
believed with intense light, as that of the sun, and the different 
colours cannot be procured at all; for such an intense light 
rapidly changes the outermost layer into black iodide, and then 
reconverts it into the coloured variety ; while the parts which are 
more deeply situated are subsequently blackened, and conse- 
quently very different states of metamorphosis may be present, 
even in the very thin layer which is affected by light. In the 
case of the vapours of mercury we may also see very different 
effects, according to their elasticity, either negative or positive 
images, pictures which remain fixed, or may be easily and com- 
pletely wiped off. The most general axiom that I can propose 
with reference to the influence of the above-mentioned causes 
is, that by their means the affinity of all bodies for vapours is 
modified, so that they are precipitated and adhere to them in a 
greater or less degree. As has been sufficiently proved, the 
modification of the substances is double, and can produce either 
a stronger or a weaker condensation. 
If I have extended the above axiom to all bodies, this gene- 
ralization will not be considered inadmissible, for the foregoing 
experiments form a whole in such a manner that a conclusion 
from one may be applied to another. For instance, if contact 
cause that peculiar modification in fluid mercury, there is no 
doubt that aqueous vapours as well as light would be able to 
produce the same if the experiment were properly made. If we 
consider the subject from this point of view, we see that the 
experiments have been extended to sufficiently heterogeneous 
bodies, and there is no visible reason why the generalization 
should not be admitted. The necessity for the surfaces of all 
the bodies being polished is no objection, because this state must 
be produced in order to render the images more visible. More- 
over, I have made experiments with several bodies that were not 
polished at all. 
