PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 485 
or in daylight. I have had occasion to make many similar ex- 
periments, and have convinced myself, that as far as regards the 
invisible rays iodized silver possesses no advantages over the 
pure metal. The only circumstance which can render the 
jodizing desirable is, that after the plates have been polished 
they may be covered with a very uniform coating of iodide. 
As this is the case I was enabled to remove a difficulty which 
I at first met with, and also to give a further proof, if such be 
necessary, of how insufficient this kind of experiment is for the 
measurement of the intensities of light. When I examined the 
action of ordinary light on pure silver, copper and glass, I was 
compelled to employ sun-light for one or two hours, in order 
then to obtain images by means of the vapours of mercury, water 
or iodine. If I had taken iodide of silver, a quarter of a second 
would have been sufficient to allow the vapours of mercury to 
exert their influence in the production of the pictures; conse- 
quently pure silver was much inferior to the iodide in point of 
sensibility. But this was not the case in other experiments ; in 
those, the success of which in my former memoir I attributed to 
contact, I had procured images in the course of ten, and after- 
wards in that of two minutes. In this case pure silver, and 
other bodies besides, proved to be very sensitive, and by no 
means inferior to the iodide. This contradiction is now ex- 
plained, and in such a manner as to allow us to suppose that it 
is very different with regard to the sensibility of various bodies 
to what we should have as yet conceived. We may consider 
that the most refrangible rays, or those whose time of oscillation 
is the least, are the most intense in regard to the effects we are 
now speaking of; that is to say, they are most fitted for com- 
mencing the action. If we extend this assumption to the in- 
visible rays, which possess very great refrangibility, it is natural 
that they should act most powerfully on pure metals, glass, &c., 
while visible light has such very little effect upon these bodies. 
In the case of an influence exerted upon pure silver, these latter 
rays stand in the same relation to the visible ones as the red or 
yellow to the blue, in respect to their action on iodide of silver ; 
consequently ordinary light must be employed in a very intense 
state, and for a great length of time, if its action is to be ren- 
dered apparent by means of vapours. If however the silver has 
been iodized, then the blue or violet latent light of this vapour 
has acted on the substance, and we may see how it might have 
become more sensitive towards ordinary light. As I have not 
f 
