W. T. WILKINSON.
PHOTO-ENGRAVING, ETCHING, AND LITHOGRAPHY.
PART I. PHOTO-ENGRAVING IN LINE.
CHAPTER I. APPLIANCES FOR MAKING PHOTOGRAPHIC NEGATIVES.
For all methods of heliographic printing a suitable photographic negative is absolutely necessary, and to produce that negative good apparatus and pure chemicals, used with skill and intelligence, are indispensable.
The studio must be well lighted, with a perfectly firm floor. The dark-room should be kept for negative work only (the room where the zinc printing is carried on being a separate room). There must be a roomy sink in front of a good-sized window covered with a non-actinic medium (yellow, not orange), such as two thicknesses of golden fabric, so as to have plenty of light to see the progress of development by.
The copying camera must have a long bellows and should not be less than 12 by 10, fitted with rack-work, so as to be able to get absolute sharpness in focussing.
It will be a great advantage, and aid in adjusting the position on the focussing screen of the drawings to be copied, if a vertical line be drawn, and another horizontally, so that they bisect each other exactly in the centre; then from the centre, mark and number each inch and half inch upon each of the four arms of the cross; it can then be seen at a glance not only that the picture is in the centre, but also that it is about the right size. {10}
The dark slide must be light-tight, and made strong rather than for portability.
The lens should be rectilinear, and should be capable of covering the largest size plate the camera will take quite sharp to the corners. For a 12 by 10 plate a lens with an equivalent focus of about 18 inches will be best.