462 PROFESSOR LUDWIG MOSER ON INVISIBLE LIGHT. 
exposed to the vapours of iodine, a very distinct image was 
formed ; it would consequently be a discovery if we could find 
any body which does not possess self-luminosity, or in which it 
is present in so small a degree as to escape our observation. 
I have made all the experiments on the self-luminosity of 
bodies in so-called darkness, in order to remove the objection 
that any influence might have been exerted by extraneous light. 
But this is not necessary, because we have seen that there are 
characteristic tests for invisible light (vide the treatise on Latent 
Light which follows), and by means of which it becomes impos- 
sible to confound it with visible light. The two kinds differ, 
however, from each other, only as violet does from red, in a 
physical sense, and the method which teaches us the difference 
between these two last-mentioned colours extends also to the 
visible and invisible rays. With this exception, there is no effect 
that cannot be produced by one kind of light just as well as by 
the other. These characteristic tests are however not necessary, 
and we need not be so careful in excluding daylight in our ex- 
periments with the peculiar rays of light, inasmuch as it cannot 
produce any additional effect if they be properly conducted. I 
have already shown that if we form images by means of common 
light (including the dark rays of Ritter) on clean surfaces of sil- 
ver, copper, glass, &c., the sun requires to act for one or two 
hours; and I can add, that if we allow the picture of a camera 
obscura, directed towards houses illumined by the sun on a fa- 
vourable day, to act for twelve hours on any of the above-named 
plates, and afterwards expose it to the vapours, scarcely a trace 
of an image is produced; consequently, if a body can depict 
itself upon metals by means of its peculiar light in the course of 
ten or even two minutes, it is evident that common light, even 
of great intensity, cannot increase the effect. 
2. As yet I have caused the invisible rays to act on gold, sil- 
ver, German silver, copper, brass, iron, steel, zinc; moreover, 
on yellow and on blackened iodide of silver, on copper which 
has become tarnished of a purple colour, on glass, porcelain, 
mica, ordinary japanned plate, and even on mercury. In order 
to employ the latter I covered a pure plated sheet of copper with 
a thick layer of it, so that the surface consisted of fluid mercury. 
The bodies which I caused to act on it were of iron and horn, 
and also the steel stamp of a medal. The mercury did not ex- 
hibit a trace of an image after this treatment, but it was pro- 
duced so distinctly on exposing it to the vapours of iodine, that 
