482 PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 
mercury render them visible. It may be said that it is very 
fortunate for the science of Daguerreotype, that this vapour does 
not possess blue light, for in this way it would not be possible 
with the visible rays of light. From what has been said above, 
it is also quite evident that the latent colour of this vapour can- 
not be white. Moreover it cannot be red; for if an iodized plate 
be allowed to remain only a short time in the camera obscura, 
no image is produced on it by means of mercurial vapours, 
although red glass soon renders it apparent. I will only men- 
tion one of the numerous experiments that I made on this 
point ; on a day when the time necessary for the production of a 
good Daguerrian image was ten minutes, I placed the plate of 
silver in the apparatus for only half a minute to one minute, 
and on covering it with a red glass I afterwards obtained a per- 
fect negative image. I then exposed it to the action of light in 
the camera obscura for one, two, and three minutes, and after- 
wards treated it with mercurial vapours, but then no picture was 
produced. A similar plate was allowed to remain for three days 
over mercury, which was several times heated to 60° R. and 
upwards, and yet only a uniform layer of condensed mercurial 
vapour was produced, and no trace of an image. 
At different times, and when I had become acquainted with 
the general behaviour of vapours, I was much astonished at this 
result of many experiments; now that we consider the latent 
colour of these vapours, it will be evident that nothing else could 
take place. To return once more to Daguerreotype, we cannot 
help lamenting that the latent colour of the vapour of mercury 
is not red; in that case its images would be formed in a much 
shorter space of time, and would not probably possess so gray 
a tint. 
The colour of the latent light of the vapours of mercury is yel- 
low; all phenomena with which I am acquainted agree with this 
supposition. In that state of the image on iodide of silver in 
which it can be destroyed by yellow rays, the vapours of mer- 
cury cause its appearance; but in the earlier stages the image 
produced by ordinary light is “levelled”? both by the yellow 
rays and the vapours of mercury. Yellow light very easily pro- 
duces blackening, and the same is the case with the mercurial 
vapours ; and indeed it appears as if the usual gray or brownish 
tint of Daguerrian pictures, particularly when exhibiting much 
detail, depends upon a partial blackening caused by the mercury. 
