PROFESSOR LUDWIG MOSER ON INVISIBLE LIGHT. 463 
the inscription of the stamp could be read with the greatest ease. 
The above-mentioned substances differ only, as regards the 
experiment, in the degree of polish which can be given to them, 
or which they naturally possess. Mica produces the images 
with great exactness, as does also well-polished copper. I have 
in my possession a plate, the surface of which consists of brass, 
iron, copper and zinc, and upon which I allowed an engraved 
plate of agate to act. When exposed to the vapours, all the 
metals exhibited their respective parts of the image; but it was 
less distinct upon the iron, which was not well polished. A 
good plate of steel gave excellent images. In all cases where it 
was possible I first used the vapours of mercury of the usual 
tension, and then those of iodine, a process which has proved to 
be advantageous even with japanned plate. [Vide the following 
treatise.| We cannot therefore doubt that light acts uniformly 
on all bodies, and that, moreover, all bodies will depict them- 
selves on others, and it only depends upon extraneous circum- 
stances whether or not the images become visible. 
3. Among these circumstances may be reckoned the divergence 
of rays, which takes place with invisible as well as with visible 
light. It prevents us from placing the bodies at too great a di- 
stance from each other, if we wish to obtain distinct images. At 
first I removed the silver plate only one-fifth of a line from the 
body which was to be depicted, afterwards I increased the di- 
stance to a whole line. Even when this was the case, by the 
use of suitable objects I obtained distinct images, parts of which 
were however confused, as might be expected. 
If we consider the circumstances under which the pictures of 
the substances are produced, we see that engraved plates are 
much more fitted for the purpose than raised figures, which 
cannot be advantageously employed. I succeeded very well with 
engraved plates of agate or wood, brass or iron stamps, and 
printers’ types. In using the latter a suitable distance is found 
to be advantageous, for it changes the sharp lines of the object 
into a more uniform shadow on the image. Black writing on 
white paper depicts itself so as to be visible, but I have never 
seen it very well executed. The same is the case with mosaic 
work, the single divisions of which are represented with surpri- 
sing accuracy, while, on the other hand, I never could render 
the figures, which differ only in their coiour, very visible to the 
eye. I will remark, as a general rule, that it does not neces- 
