478 PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 
portion of blue and violet rays contained in it. In those cases 
where the blue rays do not invert the negative image because it is 
in a too early stage of development, the light of the sun or day is 
also not capable of effecting it; consequently in this case the un- 
decomposed light acts like the blue and violet rays, and unfortu- 
nately this fact is general. After manifold experiments I have not 
been able to discover anything in which the action of undecom- 
posed light on iodide of silver differs from that of the blue and 
violet, or even of the dark rays of Ritter. This is the circumstance 
which I mentioned above, and which will probably render it ne- 
cessary to examine other substances besides iodide of silver. How 
extremely disagreeable this circumstance is, will be seen hereafter 
in the determination of the latent colour of vapours of water. 
If we assume the proposition concerning the identity of the 
action of all colours on iodide of silver as proved, it does not 
appear to advance, but rather to hinder the solution of the prin- 
cipal question, viz. to determine the refrangibility of light from 
the action it has produced; for if the condensation of mercurial 
vapours produces an action similar to that of light, it would not 
be possible to determine the latent colour of these vapours, be- 
cause all colours produce the same effect, and only differ in their 
degrees of intensity, for which however we possess no means of 
measurement. Such is then the case, and we should thereby 
be compelled to forgo the most important part of the examina- 
tion of latent light, if I had not succeeded in finding a peculi- 
arity in the consecutive action of ¢wo kinds of differently refran- 
gible rays, by which the determination of the latent colour of 
vapours is already possible within tolerably narrow limits, and 
which leads us on towards the solution of the more general pro- 
blem on which we are engaged. I shall denominate this pecu- 
liarity the levelling (Nivelliren) ; the facts referring to it afford 
a purely empirical, but at the same time invaluable, means of 
distinguishing different colours in their action on iodide of silver. 
The theoretical consideration of this peculiarity belongs, as it 
seems to me, to another class of interesting facts, which I hope 
to bring together at another opportunity. 
Bring an iodized silver plate into the camera obscura, and 
leave it there until the image has arrived at the state which may 
be generally called “that of Daguerre.” No trace of the image 
is seen on the plate, as is well known, although it is present and 
may be brought out by means of mercurial vapours. If this 
