GROUND GLASS. A sheet of ground glass is generally used for the focussing screen of the camera. It should he as finely ground as possible, and the best way to secure this is to use plate glass which has not received the final polishing in the manufacture. At sheet of plate glass may be ground by hand, by rubbing with a smaller piece of thick plate, and the finest powdered emery moistened with water between.. It takes three or four hours continuous labour to do this properly. The last and finest grinding may be given with a mixture of oolcothar (red oxide of iron, iron rust,) and water, instead of emery.
The ground glass screen should be placed in the camera with the ground side next to the lens ; and it will be found very useful to mark two diagonals across it, and also one central horizontal line, and an odd number of vertical lines, with a lead pencil.
A good substitute for a ground glass focussing screen is to coat a piece of plate glass with spirit varnish applied to a cold plate, and allowed to set without heat. When dry, it forms a semi-opaque film upon the glass. Benzole varnish with a little wax in solution
also answers the purpose.
GuArAcubt. A resinous exudation from the Guaiacum qfficinale, a lofty tree, native in Jamaica and St. Domingo. It is soluble to the - - extent of 90 per cent. in absolute alcohol. If a piece of paper be dipped in tincture of guaiacum, and exposed to the ptismatie spec trum, it is oxidized, and becomes of a green tint in the violet rays ; but in .the red rays the green colour is said to be destroyed..
GUM. Gum is a substance which trout, lamely in the vegetable kingdom, and is characterized by forming a .viscid adhesive solu tion with water, which precipitates by the Addition of alcohol a white magma. It occurs in the form of an exudation upon the bark of trees, and collects into drops, which gradually harden by exposure.
There are two principal 'modifications of gum, one of which is well represented by gum arabic, and the other by gum tragacauth.