AND ON THE ACTION OF LIGHT ON BODIES. 427 
very different from that of Becquerel. The violet and blue rays 
are the only ones which affect the common iodide of silver, al- 
though the action is at jirst so trifling that it cannot be perceived. 
Exposure to the mercurial vapours proves, however, that a cer- 
tain change has taken place. We can distinguish two steps in 
this modification of the iodide. In the first it is acted on in 
such a manner that afterwards the red and orange rays affect it 
just the same as the blue and violet. The yellow rays have not 
yet the power to act, for if it be taken out of the camera obscura 
too soon, yellow glasses produce no action. In the second stage 
of modification the yellow and green rays also have power to 
act. This is about the moment when the modified iodide of 
silver has acquired the power of condensing the vapours of mer- 
eury. At this time then, ail colours can exert their influence on 
the iodide, and consequently it would not be correct to speak of 
rays in the spectrum which act only chemically. What follows 
will form a still stronger objection to this general opinion. 
The action of the violet and blue, and at a later period of all the 
other rays of the spectrum, is to blacken the yellow iodide of sil- 
ver. If this state has once been produced, the blackened salt is no 
longer affected by any of these rays; their influence is at an end. 
There are many opinions concerning the nature of this black 
_ body; some consider it as pure metallic silver, others as a lower 
~ jodide of silver, a sub-iodide; while others contend that even 
“when the light has exerted its influence for a length of time it 
produces only an isomeric change in the iodide a silver; this 
view appears to me the most probable. It will hereafter be more 
fully demonstrated that this black body is not pure silver. At 
present it is sufficient to mention the fact that the yellow rays 
produce a positive out of a negative image, that is, they de- 
colorize the blackened silver, which is sufficient to prove it is 
not pure silver, with which no such phenomenon can be ob- 
served. It would be highly desirable to have this curious body 
better examined. Now, as the violet, blue, red and orange rays 
no longer act on the blackened silver, but the green and yellow 
rays do, we have here another proof that there is no reason to 
Speak of the peculiarly chemical action of one group of rays com- 
pared with the other, and 1 consider my first postulate (No. 1.) as 
| fully proved. If we consider the preceding statements, we shall 
md, that, in order to examine the peculiar action of the green 
id yellow rays, it is not necessary to use coloured glasses, inas- 
! Q2F2 
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