Diagrams, experimental equipment and spectrum by Shelford Bidwell Three diagrams, labelled figures 1-3, showing the setup of the equipment used in Bidwell's investigations into the recurrent images following visual impressions, a spectrum and a chart of intensity of luminous sensation against time. Subject: Optics / Physiology Prints of these diagrams were published in volume 56 of the Proceedings of the Royal Society alongside the paper 'On the recurrent images following visual impressions'. 1894