PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 479 
plate be now exposed to day- or sun-light, strange to say, no 
image is formed, but the plate is only uniformly blackened. In 
such a case I should say that the undecomposed light has 
“levelled” (nivellirt) the image, for there was no apparent dif- 
ference between the affected and unaffected parts. What is still 
more extraordinary is, that if the plate be allowed to remain still 
longer in the camera obscura, so that a negative image is formed, 
it is also washed out by undecomposed light, and only remains 
fixed when the negative image has reached one of the higher 
stages of development, and in this case it is destroyed neither 
by undecomposed light nor by any colour whatever, but it re- 
mains for some time unchanged, and then passes into a positive 
image, as has been already described. This destruction of the 
images by undecomposed light does not depend upon the simul- 
taneous action of the differently coloured rays of which it is 
composed, because blue and violet light, and partly the green, 
are capable of performing the same thing. The first two will 
always destroy the image if it is in the Daguerrian state, or even 
a negative one, if not very strongly formed, and indeed just as 
well as day- or sun-light. 
If we consider the above-mentioned fact, keeping in mind the 
axiom which we have proposed concerning the action of the 
several colours, we shall arrive at an idea of the phenomenon of 
levelling, which agrees sufficiently well with the reality. When 
the ordinary rays in a camera obscura have only acted a short 
time on the iodide of silver, then rays of any degree of refrangi- 
bility are capable of levelling the image, which is always sup- 
posed to exist, although means have not been employed to ren- 
der it apparent. If the rays have acted for a longer time, then 
all of them except the red are able to level the image, and this 
is because the rays of this colour differ sufficiently in refrangi- 
bility from those which had commenced the action. If the plate 
remains still longer in the camera obscura, we may suppose that 
the effect has been produced by still more refrangible rays than 
the blue, violet, or dark parts of the spectrum, and now a uni- 
form illumination of the image with yellow light is not capable 
of destroying or levelling it, &c. Finally, if the plate has re- 
mained several hours or even days in the camera obscura, then 
the blue and violet rays are not capable of levelling the image, 
as has been shown above. We will now extend the discussion 
to the invisible rays, which have been proved to possess the 
