CLEARNESS. This quality in a photograph implies the absence of any action except what is induced by the impact of light, the exact amount of exposure, both as to the intensity of the light and the duration of its action, the exclusion of all light except that reflected by the object which is photographed, perfect optical arrangement so that the image is sharp and evenly illuminated, a uniformly sensitive surface to receive the impression, the arrangement of light so that the illuminated object to be taken appears perfectly modelled, and clean, uniform and smart manipulation. The things to avoid are dirty plates, impure chemicals, too weak or too strong solutions, or those of unknown or inconstant strength, too much heat or cold, bad water, dirty hands or frames, and unclean vessels which cause action independent of the light ; lenses with foci not coincident or with no sharp focus, and of too short focus, and cameras shaky or not light tight, or not ad,justed to the focussing glass, or open in front to rays from any objects beside the one to be photographed, or deficient in means of absorbing the light scattered by the lenses or the mounting, which cause an imperfect image ; illuminated smoke or vapours between the object and lens, bright back-ground or sky to the picture, and dew or grease on the lens, which introduce into the camera diffused light that veils the picture ; feeble light, or too intense, or not falling up on the object so as to throw out the relief, light coming too obliquely from parts of the object, and exposure too long or too short, which injure the purity of the modelling ; and careless, or unequal, or dilatory manipulation which makes one part of the plate more sensitive or develope more quickly than another.