476 PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 
of the duration of oscillation with the continuation, and the 
similarity in the action of all rays in this respect. 
I will now turn to the peculiar power which the yellow and 
green rays are said to possess, of converting the blackened iodide 
of silver into its primitive coloured variety*, of which many 
examples were offered in my first treatise. The most convenient 
way of making these experiments is to allow a negative picture 
to be produced in the camera obscura, i.e. one in which the 
bright parts of the object are dark, and then to expose it to the 
light of day or the sun, covered by a green or yellow glass. A 
positive image is produced, for the black iodide is decolorized, 
and the unaffected parts are turned black. Gaudin made his 
experiments in this mannery. 
I will now show that this singular property does not apper- 
tain to the yellow or green rays alone, but just as well to all the 
others, and that in this respect there is no difference between 
the colours except in point of time. A plate, in that state in 
which it is generally exposed to the mercurial vapours, is ex- 
posed to day- or sun-light under a red glass; a negative picture 
is produced. If in its first stages it be placed under a yellow 
glass, it becomes positive ; if under green or blue, it becomes not 
positive. If, however, the negative image is further advanced, 
i. e. if it has remained longer under the red glass, then the green 
rays will render it positive, and finally also the blue and violet. 
This result is so interesting, that I will mention one or two of 
the experiments performed. An iodized silver plate was allowed 
to remain in the camera obscura for three minutes, and then 
placed under a reddish-yellow glass. First of all a negative 
image was produced, which was afterwards transformed into a 
positive one. Just as the latter was about forming, the plate 
was put under a violet glass, and it was then very quickly de- 
veloped. Moreover, on a tolerably clear day, an iodized silver 
plate was left in the camera obscura eleven hours; a beautiful 
negative image was visible, the light parts were greenish, and 
* It is necessary to remark, that by the expression “ coloured iodide,” I only 
mean to represent the phenomenon, but by no means to express any opinion 
as to the chemical nature of the body so produced. It may be proved that this 
state of the iodide of silver at this particular period, if considered physically, 
may also be attained by simple bodies, as silver, iron, &c., by means of continued 
exposure to the action of light; only this subject requires a more extended ex- 
amination. 
+ Comptes Rendus, Juin 1841. 
