aperture of the lens. If the number on this card is best defined on the negative the coincidence of the foci is satisfactory, but if another number is sharpest in the negative the lens requires a correction after focussing. When only one lens is to be used on a camera it is usual to so alter the position of the ground-glass in the setting that the figure which is best defined in the photograph shall show as sharpest on the screen.
FOCUS. Treated of as sub-head under focal length.
Focussing Cloth. For home use nothing is better than black cotton velvet, but in travelling thin black india-rubber cloth or mackintosh cloth is perhaps better, as it can be used in many ways to protect apparatus against rain and wet.
Focussing Glass, Focusser, or Focussing Magnifier. A
small magnifying eye-piece, used to obtain microscopic sharp-ness of focus upon the ground-glass.
Focussing Screen. The ground-glass upon which the image formed by the lens is seen. The best glass to use for this purpose is obscured patent plate, obtained by grinding patent plate with very fine emery. To make a screen of ground-glass rather coarse emery powder (that sold as No. ioo) should be made into a paste with water, and lightly rubbed over a piece of glass, using a small painter's muller, or any other convenient flat surface for grinding, till the surface begins to show signs of abrasion, when the coarse emery should be completely washed off, and the finest "flour" emery obtainable used till the glass is obscured enough. Two pieces of glass can be ground at the same time by fixing a small cork, or anything that will serve as a handle, on to one piece of glass, with a piece of cobbler's wax or pitch, and using the one to grind the other. Should the focussing screen be accidentally broken, a piece of plain glass daubed over with putty, or coated with a paste of flour and water, or at a pinch a fine cambric handkerchief, may be used. The following solution spread on glass has been suggested as a substitute for the ground-glass : -
White wax............... 120 grs.
i-*incr,., ••« a«« ••• ••• ••• 1 oz.