Flatness Focal Length
Flatness. A want of vigour and contrast in the negative and resulting prints, due to over-exposure, or to the use of too strong or too weak-a developer.
Fluorescence. See Radiography.
Fluorhydric, or Hydrofluoric Acid. HF=2o. Is obtained by heating fluor-spar with sulphuric acid in a leaden or platinum retort, connected with a receiver of the same metal. It is a colourless gas, very soluble in water, and condensing at 20 o C. into a mobile fuming liquid, which boils at 59 o F- It has the peculiar property of dissolving glass and other silicates, and for this reason leaden or platinum vessels are used to prepare it, and the aqueous solution is stored in india-rubber bottles. Extreme care is necessary in handling it, as even when dilute it causes very painful ulcers when applied to the skin, and dissolves the nails. It has several photographic uses, all of which depend on its solvent action on glass. Hydrofluoric acid has been recommended for detaching the negative film from glass plates, for preparing a film negative. See Negatives, Stripping of ; also Hyalography.
Fluorine. F = 19. A non-metallic element never met with in a free state. Fluorine has only recently been isolated.
Focal Aperture. See Aperture, Focal ; also Diaphragms.
Focal Length, Focus, or Equivalent Focus. The latter term is applied specially to a compound or doublet lens. It is the focus of parallel rays entering the lens, and is thus called from the fact of the image formed equalling in size that formed by a single lens. It is extremely important that every photo-\ grapher should know how to find the focal length of his lens, because upon this depends the determination of several factors. Before deciding, however, on the best method of measuring the focal length, it is necessary to know where we are to start measuring from. If we take a single or landscape lens we shall find that there are three or four points which we can measure from. Thus we may measure from the plane of the diaphragm, from the front surface of the lens, or from the centre of the lens, or from the posterior surface of the lens; and each point will