162 Prof. Draper on the Detithonizing 
and vapours, which tends to bring about the same results. So 
powerfully indeed does this cause operate, that, as I have said, 
for a length of time I attributed all the pheenomena to it. 
- I proceed now to describe some simple experiments which 
will bring this matter clearly before the reader. 
Take a bromo-iodized silver plate, expose it to the light of 
the sky or lamplight for a length of time sufficient to brown 
it sensibly and uniformly all over. In this state, if it were 
placed in the vapour of mercury, it would solarize or blacken 
in every part. But, before mercurializing, treat it as follows. 
Lay upon it a fragment of glass, a piece of metal, or any other 
object; immerse it for a second or two in a box containing 
the vapour of iodine; withdraw it, remove the little object, 
and mercurialize forthwith: and now you will find a per- 
fectly formed, black, spectral impression of the object, what- 
ever it was, powerfully brought out by the mercury vapour ; 
but on all those parts to which the iodine vapour has had ac- 
cess, the mercury will not adhere, but the phenomenon will 
take effect as though the plate had never been exposed to the 
light, except on those portions on which the object, whose 
spectral image appears, was laid. 
From this it would seem that the vapour of iodine has the 
quality of detithonizing a surface that has been changed by 
light. 
The same process may be conducted so as to give a still 
more striking result. 
Employing a prepared bromo-iodized plate as before, ex- 
pose it to any uniform source of light for such a length of 
time that if it were mercurialized it would whiten uniformly 
and exhibit the aspect of an ordinary white Daguerreotype. 
Treat it as before, by placing on it any object, pass it into the 
vapour of iodine,—remove the object, and mercurialize: and 
now a spectral appearance of that object, of a dense white 
aspect, will emerge, the remainder of the plate being quite 
black and in the condition of the shadows of a Daguerreo- 
type, that is, as though it had never been exposed to the light. 
In order to obtain a clear idea of what passes under the 
foregoing circumstances, I made the following trial. 
Upon a plate prepared and deeply tithonized, as has been 
said, I Jaid a double convex lens of about two inches focus, 
and exposing the plate with the lens upon it to the vapour of 
iodine, and then removing the lens, I mercurialized. A deep 
blue spectral image emerged, of less diameter than the lens, 
but like it of a circular form, its circumference being marked 
by as sharp a line as if it had been drawn by a pair of com- 
passes. Indeed, it looked as distinct and as sharp as if a blue 
wafer had been laid on the plate. 
