The Dictionary Of Photography

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

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paper, 16x12 in., my usual size for enlarging from a quarter-plate negative, being pinned to the easel top and bottom, a wedge-shaped piece of wood was passed beneath one end of the paper till the margin of the picture was equally sharp with the centre. The distance to which the margin was advanced towards the lens was found to be 1 in. Two pieces of wood were then prepared, 12 ins. long and 6 ins. wide. They were then planed down so as to form two wedge-shaped pieces 1 in. deep at outer andat inner edge. These were then screwed down upon
the easel, and to. them a thin piece of cardboard fixed, thus forming the required curved surface. Upon the picture being sharply focussed the sheet of sensitised paper is pinned. With Ilford rapid paper and the artificial light I use, which I shall presently describe, and using an R.R. lens, 8-in. focus, at full aperture, an exposure of from 8 to 10 seconds suffices to enlarge from a quarter-plate up to 16x12. If a longer exposure be necessary, as it is in enlarging from landscape negatives, into which skies have to be introduced, the lens can be stopped down to any extent, the exposure being calculated upon the well-known ratios of the squares of the diameters of the stops, or a slow paper can be used. The artificial light I use is the Welsbach incandescent gaslight, said to be 16-candle power, and I find it most convenient. It is attached to the nearest gas-bracket by a flexible tube, and it can be lighted and extinguished in a moment."
Having obtained a critically sharp image, the next point to decide is what exposure is required; and the determination is perhaps quite as difficult and equally important as in negative making. Various methods have been suggested, but before entering upon these it would be advisable to consider the factors which govern the duration of exposure : - (1) The actinic power of the light; (2) The density of the negative; (3) The intensity ratio of the stop ; (4) The number of times of enlargement, or the distance between the lens and sensitive surface ; (5) The sensitiveness of the material on which the enlargement is made.
(1) The Actinic Power of the Light. The only satisfactory method of determining this is by the aid of an actinometer; and the most satisfactory, and, in fact, the only ones to use are those