A study is made of the problems in preparing cemented cube beam-splitters which are absorption free, spectrally neutral and provide equal reflectance and transmittance for unpolarized light. Several designs are discussed and it is concluded that the most practicable is a six-layer coating of zinc sulphide and magnesium fluoride enclosed in a crown glass cube. When used in a visual instrument, the reflected and transmitted images formed by this beam-splitter are sensibly equal in brightness and colour.