484 PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 
The colour of the latent light of the vapour of iodine is either : 
blue or violet; for this vapour levels every image in the Daguer- 
rian stage without fail, and even a partially developed negative 
one; consequently this vapour does not contain either red, green, 
or yellow light in a latent state, but behaves just like blue or 
violet glasses. When however the negative image is strongly 
formed, as for instance by allowing the iodized plate to remain 
ten hours in the camera obscura, then the vapour of iodine is 
not capable of levelling it even by the most continuous action. 
The image is always visible, notwithstanding the manifold colours 
which are produced. 
While on the one hand the vapours of iodine easily and com- 
pletely destroy images produced by ordinary rays of light, they 
bring out those formed by the invisible rays just as well as blue 
or violet glasses, and in this respect may be very advantageously 
employed. I placed an engraved brass plate, an iron body, a 
small plate of silver, and a ring of black horn upon an iodized 
sheet of silver, and allowed them to remain there for several 
hours. The plate afterwards exhibited nothing whatever, but 
on being exposed to vapours of iodine until the iodide acquired 
a blue colour, the images became plainly visible, and when placed 
under a blue glass were developed with all their finer details. 
My former treatise contains several other proofs of this kind. 
If we consider the nature of these vapours, we must conclude 
that the iodizing the silver plates does not under all circum-. 
stances increase their sensibility. It is true, that if we operate 
with ordinary rays, as in a camera obscura, it is then very neces- 
sary to iodize the silver, whereby it is exposed to the action of 
the blue or violet light which is being set free. But this opera- 
tion cannot be of any service when we employ invisible rays ; 
it must in this case be as much a matter of indifference whether 
the plate be previously iodized, as it is in ordinary experiments 
whether the iodized plate be exposed to yellow or red light be- 
fore being introduced into the camera obscura. It may at first 
appear somewhat singular to allow the light to act first on the 
pure silver and then the iodine, in order to obtain a visible image; 
but the idea proceeds from the nature of the invisible rays and 
of the vapour of iodine, and it is confirmed by experiment. 
Bodies of brass, silver, horn, glass, &c. were laid upon a plate of 
pure silver only for two minutes. On introducing it into the 
vapours of iodine the images became visible although only weak ; 
they became much stronger and better defined under blue glass. 
