the plate while this is held over a spirit-lamp. It is then again
rinsed with pure water, and is ready for its frame.

2. THE PHOTOGRAPH.--Just as we must have a mould before we can make a
cast, we must get a _negative_ or reversed picture on glass before we
can get our positive or natural picture. The first thing, then, is to
lay a sensitive coating on a piece of glass,--crown-glass, which has a
natural surface, being preferable to plate-glass. _Collodion_, which is
a solution of gun-cotton in alcohol and ether, mingled with a solution
of iodide and bromide of potassium, is used to form a thin coating over
the glass. Before the plate is dry, it is dipped into a solution of
nitrate of silver, where it remains from one to three or four minutes.
Here, then, we have essentially the same chemical elements that we
have seen employed in the daguerreotype,--namely, iodine, bromine, and
silver; and by their mutual reactions in the last process we have formed
the sensitive iodide and bromide of silver. The glass is now placed,
still wet, in the camera, and there remains from three seconds to one
or two minutes, according to circumstances. It is then washed with a
solution of sulphate of iron. Every light spot in the camera-picture
becomes dark on the sensitive coating of the glass-plate. But where the
shadows or dark parts of the camera-picture fall, the sensitive coating
is less darkened, or not at all, if the shadows are very deep, and
so these shadows of the camera-picture become the lights of the
glass-picture, as the lights become the shadows. Again, the picture is
reversed, just as in every camera-obscura where the image is received on
a screen direct from the lens. Thus the glass plate has the right part
of the object on the left side of its picture, and the left part on its
right side; its light is darkness, and its darkness is light. Everything
is just as wrong as it can be, except that the relations of each wrong
to the other wrongs are like the relations of the corresponding rights
to each other in the original natural image. This is a _negative_
picture.

Extremes meet. Every given point of the picture is as far from truth as
a lie can be. But in travelling away from the pattern it has gone round
a complete circle, and is at once as remote from Nature and as near it
as possible.--"How far is it to Taunton?" said a countryman, who was
walking exactly the wrong way to reach that commercial and piscatory
centre.--"'Ba