474 PROFESSOR LUDWIG MOSER ON LATENT LIGHT, 
the invisible light has commenced its action upon iodide of sil- 
ver, the blue or violet rays possess the same continuing power 
as the red rays when the primary effect has been produced by 
visible light. Place any body on an iodized silver plate, either 
in absolute contact, or else at a very small distance, so that an 
image may be produced by the emitted rays. This body must 
be retained in its position so short a time that there is no out- 
wardly visible image produced, and no visible alteration of the 
iodide of silver. If the plate be now placed in sun- or daylight 
under a blue or even a violet glass, the picture is rapidly formed 
with all its details, while the most continuous action of the red 
and yellow rays does not produce in this case any more than a 
very slight trace of the image. This I have stated in my former 
paper, and it shows that the blue rays stand in the same relation 
to invisible light that the red do with regard to visible light. I will 
only mention one of the numerous experiments which I made 
on this point, and which all gave the same result, because I 
must return to the subject when speaking of the latent colour 
of vapours. I iodized a plate of silver, and laid upon it an en- 
graved brass plate, a similar one of silver, and an engraved stone, 
for the short space of two minutes, and in the dark. Of course 
there was nothing at all to be seen on the plate; but when ex- 
posed to the sun under blue glasses, the images appeared in a 
few minutes, and those of the metallic bodies were very perfect ; 
consequently the invisible rays of light had produced perfect 
pictures in the course of two minutes. If red or yellow glasses 
had been used, probably no image at all would have been pro- 
duced by so short an action; for rays of these colours render 
images of the invisible rays apparent only after a long interval, 
and then very imperfectly. 
Using more general expressions, we should say, if rays of a | 
refrangibility N have commenced an action, then rays of a re- 
frangibility N —n are capable of continuing it where m has a _ 
determinate value. If it is chosen very small, that is, if the rays 
which are to continue the action lie too near those which begin 
it, then no image will be produced. If, on the contrary, 2 is too 
large, then the continuation of the action will become small, and 
almost nothing as far as regards the experiment; consequently 
that which has been, not very suitably, named “continuing ~ 
force ” becomes identical with the index of refraction.. I here | 
repeat, that when I speak of refraction, index of refraction, &c., 
