Intense Ray-like Fog. Fog starting as a rule from one corner and throwing rays in different directions indicates leakage of light in the dark slide or its junction with the camera. 2 Marginal Fog. Fog forming a black or dark grey border and ailing off towards the centre sometimes occurs on plates or films which are very old or which have been kept under bad conditions.
Parts of the Plate do not Develop, or Develop only Slowly. The surface of the emulsion has not been fully covered by the developer, prob ably because too little solution has been used and this has not been well stirred ; possibly, also, to the dish not being level.
Black Lines Covering the Whole or Part of the Plate. During the preparation for taking a photograph, or when carrying the camera, an image of the sun is formed on the sensitive surface by a tiny hole in the camera itself. This acts like a pinhole, and moves in all directions according to the movements of the camera. Parts of these lines may be reversed by solarization.
Spreading of the High-lights into the Image of Shadows. Use of sensitive material not protected against halation (§§ 231-238) for photographing very contrasty subjects.
Black Spots Irregularly Distributed over the Image. Black spots of various shapes and sizes may be due to specks of some substance which causes fog or accelerates development. These may fall on the surface as dust or as particles from the developer itself. (Dust arising from the friction of the shutter in an aluminium dark slide, magnesium from flashlight powder, clinging to the hair or clothing of the operator.) They may also be due to undissolved particles of the ingredients in an unfiltered developing solution (crystallization may also occur in a concentrated developer when the temperature falls too low). These spots sometimes have tails. which are usually vertical, when tank develop ment has been employed. When dust has fallen on the surfaces before exposure to light, the black spots fiequently have a transparent centre.' Pseudo-reticulation. Dark lines somewhat like the meshes of a net have been caused in many cases by development in a bath which is not rocked or in a dish which contains only a small quantity of developer.
Edge and Streamer Markings. When develop ment is done in a very shallow layer of solution in an unrocked dish, there are sometimes produced dark margins on the dense regions of the image and light margins on the adjacent lighter regions. This is due to diffusion ex changes between the active developing agent and the products of reaction. For the same reason, when vertical tank development is em ployed, light streamer markings, extending below the denser parts of the image, are produced.
Denser Bands Recurring Periodically in a Length of Film Developed on a Rack. These bands appear on the parts of the film opposite the end cross-bars of the frame. In these regions development is more rapid than in the vertical regions along which the products of the develop ing action stream. Special racks with circular end cross-pieces of large diameter render this defect less likely to occur.
Irregular Stream-lines, Darker or Lighter than the Rest of the Image. The developer was not perfectly mixed when plates or films were placed in it. It may not have been sufficiently mixed when a concentrated developer has been diluted, or when the solution has been prepared from various stock solutions.
Mottling. This defect is usually the result of curtailed development of a very much over exposed negative, the developer being dilute or exhausted and insufficiently rocked.
White Spots. Several cases must be considered. Tiny areas bare of emulsion would result in completely transparent areas before fixation, and would be recognized after drying by the de pressions in the film. This defect is extremely rare. White spots, which on great enlargement are found to have sharply-defined irregular edges, usually angular, are the shadows of dust deposited on the emulsion before exposure to light. Little white spots with sharply-defined edges, circular or oval in shape, are generally due to air bubbles preventing the developer from coming in contact with the emulsion ; they frequently occur when the developer has been diluted with water taken from a high pressure supply, or with water at a lower temperature than that of the dark-room and which, on being warmed, liberates some dis solved air. Air-bells adhering to the emulsion. when the developer has not been poured uni formly on to the plate are usually of a sharply defined rounded shape, but irregular. Lines of air bubbles are sometimes formed when develop ment hangers or holders of films are used and are introduced suddenly into the developer.' White or Clear Spots, Round or Irregular, with Graded Edges. These spots are generally due to splashes of water on the emulsion, or to con densation of moisture when camera, slides, or changing box have been in a very cold atmo sphere, and are brought into a warm place. Similar circumstances sometimes produce mot tled markings, or marks with a light marginal fringe. In tropical climates, spots arising from splashes sometimes have either a dark centre or a dark ring.