452 PROFESSOR LUDWIG MOSER ON VISION, 
pours, and that of aqueous vapours, has been proved, still the 
general application of the principle to all substances and vapours 
is so important that some other instances should be given. 
There is no doubt that this proposition will be confirmed in all 
cases where the vapours produce a visible effect, and where they 
can be made to act so slowly that the proper moment may be 
easily discovered ; for it is very evident that if the action is too 
rapid and violent, the peculiar states of the surface which cause 
the production of the images will not be sufficiently clearly deve- 
loped; and for the same reason it is necessary to employ well- 
polished surfaces, a point which in my experiments considerably 
reduced the number of substances examined. 
The phenomena described above as taking place with iodide 
of silver may be observed on pure silver, platinum, copper, steel 
and black glass, by means of mercurial vapours. A well-polished 
plate must be covered with a partially excised screen, and ex- 
posed to the vapours of mercury, which, in the cases of steel 
and black glass, may be heated to 90° R. and upwards. Mer- 
cury will afterwards be found on the plates, and of course on 
the parts where the screen was cut out. If the former experi- 
ments have been understood, it will be easily seen that the parts (a) 
of the plates which have condensed the mercury are in a similar 
state to those portions of an iodized silver plate which have been 
nearly blackened by light. Just as these latter have lost the 
power of condensing mercurial vapours, so have also the parts (a) 
of the plates now experimented on. If it be a platinum, steel or 
glass plate, it may be easily proved by removing the mercury, 
either by gentle and careful rubbing, or by a very slightly raised 
temperature. If the plate be now introduced into the vapours 
without a screen, the former figures will make their appearance, 
but this time darker than the rest of the plate, depending upon 
the fact that those parts which formerly condensed the mercury 
now cause the deposition of little or none. This is, properly 
speaking, the same phenomenon as we have previously observed 
in the case of aqueous vapours, and which consists in the fact, 
that if we breathe on a plate, we find, on repeating the operation, 
that the breath avoids those parts on which it had been deposited 
in the first instance; but it was necessary to mention the ex- 
periments in this manner, on account of further experiments, 
which cannot be so well made with vapours of water. 
As the plate, when it has condensed the mercurial vapours, 
