The stop is arranged at a certain distance in front of the lens, and must not be moved. If it were brought close up to the glass, the field would not be so flat; the effect being then the same as that of a stop placed in front of a Portrait Lens, viz. simply to cut off the outside portion of the glass.[38] [38] See this subject explained in 'Photographic Journal,' vol. ii. p. 133. In taking Photographs of architectural and other subjects with vertical outlines, it is very important to have the Camera placed perfectly horizontal; since, if it be inclined either upwards or downwards, the perpendiculars will be destroyed and the object will appear of a pyramidal form, falling inwards or outwards, as before shown. It is convenient to rule the ground focussing glass with a number of parallel lines in both directions, which enables the operator at once to see that the position of the instrument is correct. SECTION III. _Mode of copying Engravings, Etchings, etc._ The engraving to be Photographed should be removed from its frame (the glass causing irregular reflection) and suspended vertically and in a reversed position, in a good diffused light. A black cloth may be placed behind the picture with advantage if any surface likely to reflect light be presented to the lens. The Camera must be fixed immovably, so as not to vibrate in the least degree when the cap of the lens is taken off. It should be pointed at right angles to the picture, and the focus determined in the ordinary way. Either a portrait or a single lens may be used, with a diaphragm sufficiently small to render the image distinct up to the edge. It is not desirable to employ too thin a Collodion, since perfect opacity of the darkest parts of the Negative is essential. An old Collodion containing free Iodine is better than a contractile Collodion, as giving a more intense and clear image. Pure Collodion iodized with Iodide of Cadmium, if found wanting in intensity, may be at once rendered fit for use in copying engravings by adding Glycyrrhizine (p. 209), until the dark parts of the negative become very opaque, and subsequently softening the excessive hardness, if necessary, by dropping alcoholic solution of Iodine into the Collodion until it reaches a straw-yellow tint. A second formula useful in iodizing Collodion for a similar purpose is as follows. Iodide of Potassium 4 grains. Bromide of Potassium 1 grain. This, with addition of Glycyrrhizine, will give a very black image. Etchings, diagrams, and drawings with pencil or ink, without much middle-tint, if on thin paper, are easily copied without the aid of the Camera, by simply laying the sketch upon a sheet of Negative Paper, exposing for a brief time to the light, and developing with Gallic Acid. This yields a Negative which is employed for printing Positives in the usual way. Full directions on this subject will be found in the Second Section of the following Chapter.