449 PROFESSOR LUDWIG MOSER ON VISION, 
a polished body a screen, part of which has been cut out ac- 
cording to pleasure, but without allowing it to touch, then 
breathe on the whole, and allow the water to evaporate, we shall 
find that on breathing on it again we shall be enabled to di- 
stinguish fully the figure of the excised parts: and still further, 
it does not require a polished body, inasmuch as dull glass ex- — 
hibits the same appearance. 
These phenomena are therefore widely diffused, and it seemed 
not altogether impossible that, if followed up, they might be- 
come of importance to the object in view ; for it is perfectly clear 
that the surfaces of the bodies must have undergone some kind of 
change. I was of opinion that the differences of temperature 
might exert some influence, and this idea appeared to be correct. 
An engraved metallic plate was warmed, and then held for about 
half'a minute on a well-polished piece of silver foil, or a clean 
mirror plate. When the plates were cold they were breathed 
on, and exhibited the above-mentioned appearances in a much 
more perfect manner ; for not only were the outlines of the body 
visible, but also the individual figures, letters, &c. and all with 
the greatest distinctness. 
Frequently silver or other metallic plates were made warm, 
and cold bodies, variously cut stones, figures of horn, paste- 
board, cork, coins, &c. allowed to remain on them for some time. 
The phenomena were all the same, and I did not observe any 
difference in the employment of these different bodies, either in 
these or in the succeeding experiments. 
As I was now able to obtain definite images on many bodies, 
I examined the plates with respect to vapours of another 
kind. A plate of silver, prepared as above, was exposed to the 
vapours of mercury, without having been previously breathed on ; 
the mercury was exposed to a temperature of 60° R. The mer- 
curial vapours did exactly the same service as the vapours of 
water, but much more perfectly : a very beautiful perfect image 
of the object employed was obtained. I now tried vapours of 
iodine, which were more particularly interesting, inasmuch as 
they combine chemically with the silver, whilst the vapours of 
mercury are only condensed by the cold plate, and adhere to it" 
without combining with the substance of the silver. A simi-— 
larly prepared plate was iodized yellow, and now indeed I saw 
in some cases the definite image of the objects by means of dif- 
ferent colouring ; in other cases the image was not yet visible. 
