The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Litmus
Luminous Paint
Litmus (Ger., Lackmus; Fr., Tournesol; Ital., Tornasole). A blue colouring matter obtained from lichens by fermentation with potash and ammonia. It appears commercially as small cakes, being made into a mass with chalk. It is used to in-dicate the presence of an alkali or an acid, the latter turning the solution red, and alkalies restoring the colour. It is usually met with in the form of small books made by steeping unsized paper in tincture of litmus.
Litre- The standard measure of capacity in the metric system ; originally intended to have a direct relation (through the metre) to the meridian circumference of the earth, but it is now, an arbitrary measure. According to our present legal definition (Statutory Rules and Orders, 1898, No. 411). " Approxi-mately one litre equals 1,000 cubic centimetres, and one millilitre equals 1-00016 cubic centimetres." The statutory definition (Joe. cit.) of the relation of the pint to the litre is " 1 litre = 175980 pints."' (See Cubic Centimetre, Metric System; also Weigh, ing and Measuring.)
Liver of Sulphur. See Potassium Sulphide.
Loss of Tone in Fixing. See Toning.
Luminous Paint and Luminous Photographs. Under the heading Latent Light we mention experiments on apsorbtion of light and its gradual return in a modified form ; this pheno-menon being generally termed fluorescence or phosphorescence : the latter term being generally used when the light is stored for a long period. A remarkably phosphorescent substance is the sulphide of calcium used as a pigment in the commercial lumin-ous paint (sold by W. C. Home, Torrens Street, City Road, E.C.). A surface painted with this material exposed to light during the day retains sufficient luminosity to be visible all night; and if a transparent positive - as a carbon print - is transferred to such a surface a self-luminous photograph is obtained. The complete extinction of the luminosity only occurs after a painted surface has been kept for several weeks in darkness; but such a surface may receive an instantaneous exposure in the camera, and if then laid on a negative plate it will impress the plate. For an experiment like this it is better to use a glass plate covered with a melted mixture of the dry sulphide and paraffin wax
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