Unpublished figure, crystals at right angles to each other by William Bird Herapath A figure in black and green ink, labelled figure 4, illustrating how 'when two crystals, at right angles to each other [...] so placed as to lie in superposition, the quadrangular or other shaped space of overlapping [is] perfectly black and impervious to light'. The figure shows further, smaller crystals overlapping a large crystal at various labelled angles. Subject: Chemistry / Optics 19 January 1854