Such a ghost we hold imprisoned in the shield we have just brought from the camera. We open it and find our milky-surfaced glass plate looking exactly as it did when we placed it in the shield. No eye, no microscope, can detect a trace of change in the white film that is spread over it. And yet there is a potential image in it,--a latent soul, which will presently appear before its judge. This is the Stygian stream,--this solution of proto-sulphate of iron, with which we will presently flood the white surface. We pour on the solution. There is no change at first; the fluid flows over the whole surface as harmless and as useless as if it were water. What if there were no picture there? Stop! what is that change of color beginning at this edge, and spreading as a blush spreads over a girl's cheek? It is a border, like that round the picture, and then dawns the outline of a head, and now the eyes come out from the blank as stars from the empty sky, and the lineaments define themselves, plainly enough, yet in a strange aspect,--for where there was light in the picture we have shadow, and where there was shadow we have light. But while we look it seems to fade again, as if it would disappear. Have no fear of that; it is only deepening its shadows. Now we place it under the running water which we have always at hand. We hold it up before the dull-red gas-light, and then we see that every line of the original and the artist's name are reproduced as sharply as if the fairies had engraved them for us. The picture is perfect of its kind, only it seems to want a little more force. That we can easily get by the simple process called "intensifying" or "redeveloping." We mix a solution of nitrate of silver and of pyro-gallic acid in about equal quantities, and pour it upon the pictured film and back again into the vessel, repeating this with the same portion of fluid several times. Presently the fluid grows brownish, and at the same time the whole picture gains the depth of shadow in its darker parts which we desire. Again we place it under the running water. When it is well washed, we plunge it into this bath of hyposulphite of soda, which removes all the iodide of silver, leaving only the dark metal impregnating the film. After it has remained there a few minutes, we take it out and wash it again as before under the running stream of water. Then we dry it, and when it is dry, pour varnish over it, dry that, and it is done. This is a _negative_,--not a true picture, but a reversed picture, which puts darkness for light and light for darkness. From this we can take true pictures, or _positives_.