Asymmetric dichroic crystals useful for light polarizing as a suspension in a set plastic medium such as cellulosic materials, e.g. cellulose acetate or nitrate, or polyvinyl compounds such as polyvinyl acetals, are obtained as an addition product which may be a periodide, by the addition of iodine to a solution of either 1 : 2-7 : 8-dibenzacridine of the general formula <FORM:0600821/IV(a)/1> and/or its reduction products, e.g. 1 : 2-7 : 8-hydrodibenzacridine or a quaternary salt of 1 : 2-7 : 8-dibenzacridine, e.g. the methosulphate, preferably so that the ratio of iodine atoms to nitrogen atoms is 2 : 1. The reaction may be effected in the presence of an acid substance which may be a mineral acid, e.g. sulphuric or phosphoric acid, or an inorganic salt particularly a sulphate or a phosphate, e.g. sodium bisulphate or sodium dihydrogen phosphate or a salt of a weak base and a strong mineral acid, e.g. copper sulphate. A neutral substance such as lead acetate may be added to lower the concentration of iodide ions by forming an insoluble iodide. In an example, 1 : 2-7 : 8-dibenzacridine and one or more of its reduction stages are prepared by melting 2-naphthol with 2-naphthylamine and adding trioxymethylene, and the product is dissolved with cellulose acetate in ethyl acetate and methanol and reacted with iodine in the presence of phosphoric acid. An additional quantity of cellulose acetate may then be added and after mixing is ready to be formed into a sheet in which the crystals are optically oriented in any desired way. Other solvents such as acetone, carbon tetrachloride and chlorinated solvents, e.g. chloroform or ethylene chloride, may be used as the reaction medium to effect the reaction with iodine. The crystals may be optically oriented in a plastic material by the application of force, e.g. by extruding through an orifice, stretching or smearing by spreading a mass of the suspended crystals upon a support by passing a blade or knife in one direction. The resulting sheet may be bonded between glass or any other hard transparent plastic material such as methyl methacrylate, and a polymerized partial acetal resin plasticised with glycerol triricinoleate may be used as the adhesive, or when cellulose acetate is used for the outer layers, they may be bonded together by moistening their surfaces with a solvent, e.g. acetone. Specifications 412,179 and 433,455, [both in Group XX], are referred to.ALSO:An addition product is obtained which may be a periodide, by the addition of iodine to a solution of either 1 : 2-7 : 8-dibenzacridine of the general formula <FORM:0600821/IV(b)/1> and/or its reduction products, e.g. 1 : 2-7 : 8-hydrodibenzacridine or a quaternary salt of 1 : 2-7 : 8-dibenzacridine, e.g. the methosulphate, preferable so that the ratio of iodine atoms to nitrogen atoms is 2 : 1. The reaction may be effected in the presence of an acid substance which may be a mineral acid, e.g. sulphuric or phosphoric acid, or an inorganic salt, particularly a sulphate or a sulphate, e.g. sodium bisulphate or sodium dihydrogen phosphate or a salt of a week base and a strong mineral acid, e.g. copper sulphate. A neutral substance such as lead acetate may be added to lower the concentration of iodide ions by forming an insoluble iodide. In an example, 1 : 2-7 : 8-dibenzacridine and one or more of its reduction stages are prepared by melting 2-naphthol with 2-naphthylamine and adding trioxymethylene, and the product is dissolved with cellulose acetate in ethylacetate and methanol and reacted with iodine in the presence of phosphoric acid. An additional quantity of cellulose acetate may then be added and after mixing is ready to be formed into a sheet in which the crystals are optically oriented in any desired way. Other solvents such as acetone, carbon tetrachloride and chlorinated solvents, e.g. chloroform or ethylene chloride may be used as the reaction medium to effect the reaction with iodine. Specifications 412,179 and 433,455, [both in Group XX], are referred to.