The Dictionary Of Photography

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

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which dates from about the same period as the French original. 11 The rays of light reflected from different bodies make a picture on all polished surfaces - for instance, on water or glass. . . . They have composed a most subtle matter, very viscous, and proper to harden and dry, by the help of which the picture is made in the twinkle of an eye. They do over with this matter a piece of canvas, and hold it before the objects they have in mind to paint - the impression of the images is made the first instant they are received on the canvas, it is immediately carried away into some dark place; an hour after, the subtle matter dries, and you have a picture so much the more valuable as it cannot be imitated by art or damaged by fire."
Glass. Early specimens are a small tablet in the British Museum, about 1445 B.C., of Egyptian make, and a goblet found in Nineveh, of about 700 B.C. The manufacture was gradually improved, till, in Italy, 58 B.C., window glass was made; and at Pompeii glazed windows were found intact. As the Roman Empire declined, the glass industry tended to where Venice now stands; and until 200 years ago Venice was supreme in the manufacture of glass generally. Now the manu-facture is widely spread all over Europe, Venice, however, retaining a first place as regards certain decorative styles. There are many kinds of glass, and the subject generally is of much interest to photographers. Crown glass is composed of a mixture of silicates of potassium and sodium, with a little calcium ; also often aluminium. Flint glass is a mixture of sili-cates of potash and lead. It is much more refractive than crown. Coloured glasses contain metallic oxides or compounds which have specific colouring properties. Copper, according to the state of oxidation, may give many tints, running through green, blue, and purple, to red ; the usual ruby glass of the shops being coloured with copper in the state of cuprous oxide. Silver gives a yellow, which appears as red when intense, the " stained red" glass recommended by Captain Abney for the dark-room window being coloured with silver. Gold gives a purplish red, useless for the dark-room window. Iron (ferrous) gives olive green, and (ferric) brownish yellow or orange, cobalt blue, uranium a greenish-yellow fluorescence, manganese an amethyst tint. Opal or milk glass contains phosphate of lime, oxide of tin, or