Finger-markings. Finger-markings appear white after contact of dry fingers with the sur face of the emulsion (and even with the backs of the plates if the latter are piled together after contact). The slightest quantity of grease deposited on the emulsion prevents the pene tration of the developer. Finger-markings will appear black if the surface of the emulsion has been touched with fingers soiled with developer, fixer, etc.
Black or White Lines. Lines, generally very fine and straight, are due to friction on the emulsion. The shutter of the dark slide may be bent ; a sheath in a changing box may be bent, and thus rubs against the emulsion of the next plate ; there may be abnormal resistance to the pulling over of films in a bent film pack ; the guide rollers may be working badly in a roll film camera, or it may be winding too tightly. Lines from these various causes are usually light on a dark ground and dark on a clear Dark Tree-like Markings. Electric discharges on an emulsion leave black brush- or tree-like markings. This trouble is hardly ever met with except on films, 2 and only then in very dry weather (especially in frosty weather). It may be due to friction or merely to the unrolling of the spool.
The Image Appears as a Positive. An image which has been considerably over-exposed may appear as a positive, altogether or partially ; in such cases it is usually fogged. An image which at first appeared as a negative may during development be converted into a positive by the action of light (white light or unsafe dark-room lighting) ; it seems in this case that the first negative image protects the underlying emulsion against fog ; at the same time the sensitivity of the emulsion is decreased by the soluble bromide already set free by development. Delayed reversal may also result from very long development in an extremely dilute developer ; the chemical fog of the unexposed regions becomes denser than the image itself. Finally, cases have been noted in which a very much under-exposed image develops directly as a positive, this reversal being favoured by an extremely short time of exposure ; it seems in such cases that chemical fog is destroyed by an amount of light insufficient to produce an image.
Double Images Ghosts. The superposition of two entirely different images is obviously caused by two exposures on the same plate or on the same part of a film. Very curious effects are
sometimes caused, however, by the superposi tion of two exposures without moving the camera ; the shutter may have been opened twice or it may have rebounded, or it may be that the sensitive surface has remained un covered for some time, and a second image has been projected by means of a hole in the front of the camera (generally due to the loss of a screw from the lens mount) ; one of these images is always so faint that only persons dressed in light colours standing near during or after the exposure appear in it ; such figures then appear as transparent ghosts. These effects are often attributed to supernatural causes.
Broad Transparent Shaded Mark, Starting from One Edge. A very large out-of-focus image of a linger, held in front of the lens during exposure, so that it obscures part of the field.
Broad Light Bands Parallel to One Side of the Plate. When a focal-plane shutter is used, the rapid passage of an opaque body in front of the lens cuts out the image (or reduces its density if only a portion of the lens aperture is masked) on the parts of the plate which happen at that instant to be uncovered by the slit of the shutter. This is frequently met with in photographs taken from aeroplanes with the camera pointing across the propeller.
432. Defects Appearing After Fixing. Milky Markings with Diffused Edges, visible on the Back of the Negative or by Transmitted Light. Fixation has been stopped too soon, and has left patches of silver bromide between the image and the support.
Transparent Marks of Irregular Shape and with Diffused Edges. Local solution of metallic silver in an old fixing bath, which contains particles of rust, or in any fixing solution con taining particles of potassium ferricyanide or other reagents capable of attacking silver. A clear mark may be produced by the prolonged action of a crystal of undissolved hyposulphite on a given tiny part of the eitiulsion.
Blisters. Blisters may be caused, particularly on films and papers, by bubbles of gas, which, instead of being liberated at the surface, are formed within the gelatine itself or between the latter and the support (immediate use of water delivered at too great a pressure ; transference without intermediate rinsing from a developing solution containing carbonate to a very acid, non-hardening, tepid fixing bath).