This box is but a trifle larger than the ordinary one, and is furnished with two sliding bottoms, working in grooves, one over the other; the lower bottom has a grooved channel, into which the side of the camera back slides; the camera back has an aperture through the side, closed by a narrow slide, and the lower bottom of the field box has a corresponding one. We now suppose the field box to have been previously filled with excited glass plates, having their sensitive sides towards the back of the box, and the box lid closed. The bottom slide is now pushed on until the aperture is in a line with any particular groove of the field box (which position is indicated by a numbered scale and index point). The camera back is then slid on to its place on the field box, so that the hinged flap is towards the front of the box, and its narrow slide drawn out. The upper slide is then withdrawn, and the box inclined, so that the plate in that groove opposite the aperture in the lower slide, may pass through into the camera back. When this has taken place, push in the narrow slide of the camera back, invert the box, and push in the inner slide; then withdraw the camera back from its channel, and expose the plate in the camera. When this is done, slide the back again into its channel, draw out the inner box slide, then the narrow camera back slide, invert the box, and the plate will then leave the camera back and pass into the field box, occupying the same groove as before. In order to get out another plate, slide the lower bottom, so that the index points to the number on the scale, as that of the groove in which the required plate is situate. Then proceed as before directed. CHAPTER IX. On a mode of Printing enlarged and reduced Positives, etc., from Collodion Negatives. To explain the manner in which a photograph may be enlarged or reduced in the process of printing, it will be necessary to refer to the remarks made at page 20, on the _conjugate foci_ of lenses. If a collodion negative be placed at a certain distance in front of a camera, and (by using a tube of black cloth) the light be admitted into the dark chamber only through the negative, a reduced image will be formed upon the ground glass; but if the negative be advanced nearer, the image will increase in size, until it becomes first equal to, and then larger than, the original negative; the focus becoming more and more distant from the lens, or _receding_, as the negative is brought nearer.