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.—­“‘Baeout twenty-five thaeousan’ mild,”—­said the boy he asked,—­“‘f y’ go ‘z y’ ‘r’ goin’ naeow, ‘n’ ’baeout haeaf a mild ‘f y’ turn right raeoun’ ‘n’ go t’other way.”

The negative picture being formed, it is washed with a solution of hyposulphite of soda, to remove the soluble principles which are liable to decomposition, and then coated with shellac varnish to protect it.

This negative is now to give birth to a positive,—­this mass of contradictions to assert its hidden truth in a perfect harmonious affirmation of the realities of Nature.  Behold the process!