IT is of great interest, particularly in cosmic ray experiments, to employ scintillation counters which have a large sensitive area. The first essential is a large transparent crystal. Crystals of naphthalene containing 1 per cent of anthracene and of size up to 8 in. in diameter and ½ in. thick have been grown, and undoubtedly it will be possible to use large crystals of other substances in the future. Here, however, we are concerned only with the optical problems which arise when large crystals are used. The aim of this note is to direct attention to a useful development in this field.