34 J. W. Draper—Distribution of 
When a polished surface of silver is exposed at common 
temperatures to the vapor of iodine, it speedily tarnishes, a film 
of silver iodide forming. This passes through several well 
marked tints, as the exposure continues and the thickness 
increases. They may be thus enumerated in the order of their 
occurrence: 1, lemon-yellow; 2, golden-yellow; 3, red; 4, 
lue; 5, lavender; 6, metallic; 7, deep yellow; 8, red; 9, 
een. 
eal these films are sensitive. Under the influence of radia- 
tions they exhibit two phases of modification. Ist, an invisible 
modification, which, however, can be made apparent or devel- 
oped, as Daguerre discovered, by exposure to the vapor of 
mercury, the iodide turning white by the condensation of mer- 
cury upon it, wherever it has been exposed to light; but 
remaining unacted upon in parts that have been in shadow. 
2d, a visible modification, which arises under a longer exposure, 
the iodide passing through various shades of olive and blue, 
and eventually becoming dark-gray. 
But though all the variously tinted films of silver iodide are 
impressionable, they differ greatly in relative sensitiveness, 
with each other. This may be very satisfac- 
torily shown by producing on one silver tablet bands of all the 
above named colors, an effect readily accomplished by suitably 
unscreening successive portions of the tablet during the process 
of iodizing, and then exposing all at the same time to a 
common radiation. It will be found on developing with 
mercury vapor that the bands of a yellow color have been the 
most sensitive, those of a metallic aspect have been scarcely 
acted on, and those of other tints intermediately. It is to be 
particularly remarked that the second yellow, numbered 7 
in the above series, is equally sensitive with the first yellow, 
numbered 2. 
rom this it appears that the sensitiveness of this form of 
iodide depends not merely on its chemical constitution, but also 
on its optical properties. The — of this different 
sensitiveness in different films of iodide becomes obvious when 
we cause a tablet prepared as just described with tinted bands 
to reflect the radiations falling on it to another tablet iodized 
to a yellow color, and placed ina camera. After due exposure 
first plate and its image on the second together, it will be per- 
ceived that the parts that have been affected on the one are 
