BRITISH GUM (See " Dextrine.") BROKEN NEGATIVES These must not be confused with cracked negatives (which see), as they are not treated in the same way. Broken negatives are generally understood to be those in which the film is broken as well as the glass. If the glass only is broken the film can be transferred to another piece of glass, but this method is not suitable when the film as well as the glass is broken, because of the danger of distorting or losing the pieces during the process of stripping. In many ways a mended broken negative is more satisfactory than a film that has been stripped and put on a new glass backing, and some photographers advocate the breaking of the film purposely when the glass is cracked in order that it may be treated as a broken negative. To mend a nega tive which has been broken into two or more pieces, take each piece and clean the edges free from dust and dirt. The largest portion is then laid upon a clean and perfectly level glass plate, and Canada balsam diluted with xylol applied to the edges very thinly with a small camel-hair brush. Xylol (or xylenc) is a coal-tar product,
and if it is not obtainable benzene can be used in its place. The remaining pieces of the negative are also touched all round the edges with the cement, and are then carefully joined so as to fit exactly. The addition of xvlol enables one to use the balsam without heating, and as it has about the same index of refraction as glass, the internal surfaces of the glass, if correctly placed together, will no longer reflect light, and the breaks will hardly be perceptible. The surface of the film is afterwards cleaned with a piece of cotton-wool dipped in benzene. Nega tives carefully mended in this way show no sign of breakage if printed slowly in the shade.