PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 487 
As the vapour of water is of such great importance, it is not my 
wish that the examination of its latent colour should appear more 
complete than is as yet possible; and I must content myself for 
the present with having proved that this latent colour is certainly 
not green, yellow, orange, or red, but that it appears to belong 
to the ordinary prismatic colours, inasmuch as [ have never seen 
it destroy an image produced by the invisible rays. 
I will conclude this Memoir with an example, showing how 
easily somewhat complicated phanomena may be explained by 
the knowledge of the latent colour of different kinds of vapour ; 
the example has also considerable practical importance as regards 
the performance of experiments of this class. If we allow in- 
visible light to act for a comparatively short time on pure cop- 
per, when it is exposed to the vapours of mercury of ordinary 
tension, an image is obtained after the lapse of some hours. 
If the plate be removed at an earlier period, there is frequently 
not a trace of the picture to be seen. If however it be now in- 
troduced into the vapours of iodine, a perfect image becomes 
apparent in a few seconds. I had accidentally discovered this 
method of operation, and described it in my former treatise, 
without however understanding the reason why the previous 
application of the mercurial vapours should be so advantageous, 
although of themselves they frequently produce no image what- 
ever. We now see that it depends on the yellow latent colour 
of the vapour. It is very evident that yellow light stands in 
the same relation to invisible light, particularly when it has 
acted for some time, that the red does to the violet or dark rays ; 
and if we first expose a plate of copper which has received an 
impression from the invisible rays to the vapours of mercury, 
and then to those of iodine, it is just the same as if we expose 
an iodized plate of silver to the uniform action, first of red, and 
then of green or yellow light. After the rationale of the process 
had been thus found, it was natural to suppose that it might be 
generally advantageous,—an idea which has been confirmed by 
a series of experiments that I made by allowing invisible rays to 
act upon gold, silver, German silver, brass, iron, steel, zinc, and 
even japanned plate. The most complete inversion of Daguerre’s 
process that is possible, and which depends upon the employ- 
ment of light of a greater rapidity of oscillation, is to act upon 
pure silver, first with light, then with mercury, and lastly with 
the vapours of iodine. 
Konigsberg, July 1842. 
