The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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action. The most suitable plates are those commercial brands, which are specially made for this work, and which are called " photo-mechanical" ; but, failing these, any ordinary slow lantern plate designed for giving black tones can be used. The lens should be a rapid rectilinear or doublet of not too long a focus, because it must be remembered that when copying at close quarters the focus is lengthened, and therefore a lens of such a focus must be chosen that the increased focus is not too long for the bellows of the camera. This increase in focus must be taken into account when estimating the factor of stop aperture for cal-culating exposure. The exposure ofTourse will vary enormously with the subject; but, as some guide, it may be said that copying a black-and-white line-drawing, such as one of the diagrams in this book, the same size, using diaphragm and a photo-
mechanical plate at noon in an ordinary room, facing a north-light window on a bright, but not sunny day, the exposure given was 40 sees., and found to be correct. For development ferrous oxalate, eikonogen, amidol, and metol are perhaps less suitable than either hydroquinone or pyro; glycin also being of value. The main point in developing is to obtain clear glass in the lines representing the blacks of the print; density of the part representing the paper is of secondary consideration because it may be obtained afterwards by intensification, though with correct exposure good density combined with clear glass can be easily obtained. Continue development as far and as long as possible, but on the slightest sign of any deposit on the lines instantly drop the plate in an acid fixing-bath. After fixation wash thoroughly, and then examine the negative; should there be any deposit in the lines reduce by the aid of the ferri-cyanide and hyposulphite reducer, thoroughly wash, soak for 10 mins. in a hypo eliminator, again wash, and intensify with the potassio-silver cyanide intensifier.
{b) Copying Photographs. In copying photographs, photo-gravures, collotypes, or any prints in which there is half-tone, it is advisable to use a medium rapidity or ordinary plate, and with prints with a considerable amount of heavy shadow it is advisable even to use a more rapid plate. The developer should be the one usually recommended for the plates, or that which the operator is in the habit of using, and soft, full-of-detail negatives should be aimed at.