200 M. C. Lea on the action of Light upon Iodid of Silver, 
The second . oct ee . generally admitted; th 
first, on the rary, is much contested. I shall therefore 
briefly state thie. pect of the saenedl series of investigations 
undertaken to arrive at a clearer view of the principles involved. 
When pure iodid of silver, isolated, is exposed to light for a 
very brief period, an invisible, or late nt image is produced, which 
by the action of a sng ‘just ready to precipitate metallie 
silver, becomes eviden 
If the action of light in producing an invisible image upon 
pure iodid of silver isolated be a chemical] one, it is not possible 
that it should be destroyed except by chemical means. 
A piece of glass supporting a film of pure iodid of silver iso- 
lated from all other substances was exposed for many hours to 
a strong sunlight. It was then placed in a dark closet for 
thirty- me hours, at the end of which time it was placed under a 
negative and e exposed to light for two seconds. On pouring a 
ccanrette over it, a clear “bright picture instantly appeared. 
Thus the action of the sun for many hours had ee an 
impression which completely disappeared in thirty-six hou 
ow if the action of light is to reduce iodid to sub- iodid, Bee 
did this sub-iodid recover its lost proportion of iodine? The 
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Ag 
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theory holds that the production of a latent image is accompa- 
nied by a reduction. he plate in question then either should 
under the action of the developer have received a deposit eg over, 
or else must have recovered its iodine. The latter cas ot 
supposable, the other alternative did not ~ place, Hanon the 
action of light could not have been chemical.” 
In some cases, the action of light pit perfectly pure iodid 
of silver, isolated from all other bodies, may produce a visible 
‘s pins ecy Pgh this pacha ni that I have seen made is one by Dr. Vogel, 
when actual chemical dec ecorpeniton indoabtely 
does take. vine: hn tomieeny is peaveréa by re in the dark. This is in the 
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case per sal 
has no tendency to oxydize by a 
itsel i a rther 
c 
< 
evelopments—the silver devel 
f the image, as is t ee 
