PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 477 
the dark of a reddish colour. On placing the glass under a dark 
blue glass, a strong positive image was formed in the course 
of half an hour, and this was not changed by exposure for seve- 
ral days. 
We now understand the manner in which the various rays 
act upon iodide of silver; they all blacken it and then re-convert 
it into the coloured variety. These processes require very dif- 
ferent spaces of time, according to the refrangibility of the rays. 
If we consider the degree of colouring of the iodide of silver, as ex- 
pressed by the ordinates of a curve, and the times by the abscisse, 
then, in the case of the blue, violet, and Ritter’s rays, the curve 
will rapidly ascend and attain its maximum, which is what we call 
blackening. From this point the curve continues to approach 
the axis of the abscissz. In the case of the yellow and red rays 
the curve takes an opposite course; at first slowly and gradu- 
ally approaching the maximum, and afterwards descending 
rapidly towards the axis. With the invisible rays we find by 
experiment that the curve which represents their action rises at 
first very rapidly, but does not so soon reach its maximum as in 
the case of the violet rays; for although, as J have already shown, 
they produce a blackening of the iodide of silver, it is only caused 
after a length of time. It is true there is no means of measuring 
the intensity of the invisible rays and of comparing them with 
violet ones; my experiments, however, have shown that the 
former are capable of commencing their action on iodide of sil- 
ver very quickly, for instance, in two minutes. If the blue or 
violet rays had commenced the action so rapidly, they would 
sooner have produced the blackening. I do not possess experi- 
ments to determine the course of the curve of the invisible rays 
after it has reached its maximum. 
I must here correct an error to which my former treatise might 
lead. In describing the action of the yellow and green rays, I men- 
tion that the same phzenomena could be obtained by means of day- 
light and the sun. ‘This is correct; but considering the theory 
then established concerning the influence of the three groups of 
colours, I was obliged to assume that this action depended upon 
the portion of green and yellow rays contained in ordinary light, 
which is not correct. It has just been shown that the blue and 
Violet rays are capable of decolorizing the blackened iodide of sil- 
yer as well as the green and yellow; and I can now add, that if un- 
decomposed light exerts this action it is owing principally to the 
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