1,146,309. Photochromic materials. NUCLEAR RESEARCH ASSOCIATES Inc. 20 June, 1966 [25 June, 1965], No. 27378/66. Heading C4S. A photochromic composition comprises mercuric bis-diphenyl thiocarbazone and a reversion accelerator comprising an amino-, nitro- or hydroxy-containing organic compound. Preferred compounds are monocyclic or fused bicyclic aromatic but may be aliphatic, aromatic or heterocyclic, solid or liquid. A U.V. absorber may be included in the composition together with dyes, spreading agents and antioxidants. The composition is employed in an organic carrier or solvent which can be liquid or waxy, thermosetting or thermoplastic resin with a plasticizer. The composition may be used as a coating, a liquid layer between sheets of glass or plastics material or as a self supporting block.