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754 experiment, because the sensitiveness of the eye to colour is sometimes very feeble, but in almost all cases sensitiveness can be strengthened by practice. In the study of optical effects, tco. the fact cannot too often be emphasised that we must know exactly what to look for and also the right spot to which the attention should be directed. We believe that an instrument called the " Chromoscope" which has been designed to show the effects referred to, can be purchased from Messrs. Georgedoi:10.1016/s0140-6736(00)48963-4 fatcat:ladbrlst65eohj5sgzvubimiiq