PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 471 
substance, and I consider the solution of the following problem 
as the groundwork for every further step in this new sphere, viz. 
as far as regards the determination of latent colour, This pro- 
blem is— 
“ Light has exerted its influence upon iodide of silver: whether 
it be visible outwardly or not, it is required to find by which 
colour this action was produced.” 
I have not succeeded in exhausting this proposition; but I 
think that I have advanced some steps towards its solution, 
which already lead to remarkable results. 
According to the definition given in my treatise on Vision, the 
influence of the differently refrangible rays on iodide of silver 
was, that the blue and violet rays commenced and continued the 
action until the iodide was blackened. This is a fact which, in 
the case of chloride of silver, has been known since the time of 
Scheele. E. Becquerel made the additional discovery that the 
red rays cannot indeed commence the action, but are capable of 
continuing it when once begun, and can carry it on even until 
blackening ensues. Finally, I had stated that the yellow and 
green rays restore the blackened iodide of silver to its primitive 
coloured state; consequently, the groups of colours, which for 
the sake of brevity may be denominated blue, red and yellow, 
exhibit qualitative differences in their action upon iodide of sil- 
ver, and this action cannot be brought into connexion with the 
refrangibility (or more correctly the length of oscillation), which 
is a very remarkable circumstance; for the action is begun by 
the most, continued by the least refrangible rays, and carried on 
still further by those possessed of a medium index of refraction. 
On further consideration, this appeared to be very incredible, 
and I shall now show that that idea was quite erroneous. 
The reality is, that the rays of every degree of refrangibility 
all act similarly on the iodide of silver, and there is no positive 
result which cannot be obtained by rays of all kinds. The only 
true difference is, that in order that the same action should be 
produced by the various rays, it is necessary to employ different 
times, and in such a manner, that if the action, measured in any 
manner, be represented as a function of the time, this function 
will have different forms for the different colours. 
As a proof of this proposition, I will first mention, that the 
iodide of silver may be blackened by coloured glasses of all kinds. 
Tn this respect there can be no doubt with regard to the violet, 
