PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 483 
Just as yellow light most easily converts the negative image, as 
obtained in the camera obscura, into a positive one, so also the 
vapours of mercury easily invert the image, if allowed to act 
for a longer period, or employed of a greater tension, as I have 
already shown in my former treatise. Yellow light produces 
the metamorphosis by blackening the unaffected iodide, and by 
decolorizing that which has been already blackened. Vapours 
of mercury exert a similar action on the parts not affected, which 
represent the shaded portions of the object ; it here commences 
its action, becomes on these parts more strongly condensed, and 
consequently alters the appearance of the picture. The latter 
“remains unaltered for several days if suspended over mercury 
occasionally warmed, a fact of which I have lately convinced 
myself. According to this we must modify the opinion which 
I expressed in my former treatise, viz. that on continuous action 
the mercury disappeared from those parts where it had at first 
been condensed. 
When yellow light inverts a negative image, a moment must 
arrive when no image whatever can be distinguished on the 
plate, and afterwards a positive one is formed. If we now re- 
move the plate from the action of yellow light and expose it to 
the vapours of mercury, a positive image is produced, which 
cannot be distinguished from that formed by the yellow glasses. 
The quantity of mercurial vapours which is precipitated on the 
plate in one kind of this experiment, is too inconsiderable to pro- 
duce any perceptible difference in the two images. 
Moreover, the behaviour of the vapour of mercury towards the 
luminous effects produced by the invisible rays, is as character- 
istic as it is conclusive of its latent colour. It never destroys 
an image produced by these rays, but it often produces it by 
very continuous action, as I have stated in my former trea- 
tise; the vapours of mercury produce the image with rapidity 
only when the invisible rays have acted for along time. The 
true reason of this was not at that time very clear, now it is 
quite evident, for the same is the case with the yellow rays. It 
does not destroy an image produced on iodide of silver by in- 
visible rays; but a long time is required even in the sun before 
an image in its earlier stages makes its appearance; and even 
then we usually see only traces of the picture, as already stated, 
probably because yellow and red glasses transmit a large quan- 
_ tity of heat, by which the iodide is altered and the experiment 
- disturbed. 
