the trees appear against a bright sky, or in any case where extreme contrasts of light and shade exist. It is caused by reflection from the back of the plate. The rays of light are scattered by the particles of silver salt, and, obeying certain laws of reflection, are reflected from the surface and back of plate. The remedies are Backing the Plate (q>.v.)t the use of thickly coated and matt-surfaced plates, and plates containing iodide of silver, the latter being advantageous, chiefly because the iodide of silver emulsion is more opaque than bromide ; but if an emulsion could be obtained absolutely transparent, there would be practically no halation. When photographing interiors, it has been recommended to cover the window with some slightly opaque substance, such as pale yellow linen or unbleached calico, so as to reduce the intensity of the light ; and, again, it has been recommended to cut small shapes of black velvet, and hang on wires in front of the camera, so as to exclude the windows themselves from the focussing screen, re-moving them only a short period before the close of exposure. When halation does mar a negative, local Reduction (q.v.) may be resorted to, or the process described under Harmonising Harsh Negatives (q.v.) may be used with great success, or the dense deposit may be partially removed by^areful rubbing down with wash-leather and methylated alcohol.
Half-Plate. The size of plate The true half-plate
is seldom known commercially as half-plate, and is
often called " double quarter."
Half-tone Blocks. The usual method of making the so-called printing-blocks or process-blocks is described under the heading, Fish Glue Process, and the undermentioned articles may be referred to as giving information which directly or in-directly bears on the subject -. - Photogravure, Galvano-graphy, Daguerreotype (etching of), Obernetter's Process, Electrotyping, Zincography, Asphalt Process, Chemi-graphy, Prism (reversing), Reversed Negatives, Types (originating by photography), and Filigrane.
Halogens. This term has been applied to the group of the four elements - chlorine, bromine, iodine, fluorine.