M. C. Lea on the action of Light upon Iodid of Silver. 203 
I think it is evident from what has been already said, that in 
this case several images are formed, superposed as it were, on eac 
other,—First, a physical image upon the iodid of silver. Sec- 
ondly, if the exposure be sufficient, an image formed of sub-iodid 
of silver produced by the action of light upon the iodid and ni- 
trate. Thirdly, there may be an image formed by the action of 
light upon the silver or its iodid in connection with the organic 
matter of the film (collodion or albumen). And, fourthly, if 
bromid or chlorid be present in the film, these may undergo 
reduction. 
The separate nature of these images, or some of them, which 
merated, was completely removed. Nothing remained but the 
third, and it is not improbable that in some of the “dry pro- 
cesses” this third source is the principal basis of the picture, 
though evidently only a subsidiary one, in the ordinary “ wet 
process.” 
Another form of this experiment consists in developing the 
picture Jefore immersing it in the solution of acid nitrate, and 
then in leaving it but a short time in that solution, so that only 
the visible picture shall be removed, and the film of iodid and 
bromid be left. a 
Here all basis for development depending upon reduction is 
removed, and the production of the picture must depend wholly 
upon the first and third of the foregoing causes to the exclusion: 
of the second and fourth. 
Without having extended my observations to the reo 
type, I may remark that this process must be ranked along with 
those in which there is a reducing substance present, and there- 
fore reduction may take place. The metallic back of the da- 
guerreotype plate stands in the same relation to the iodid that 
tannin and other so-called sensitizers do. It seems probable 
therefore that in the case of the daguerreotype, there may be 
