Report of comparative tests of roundels from an ophthalmologist's standpoint
This 1904 report by ophthalmologist Nelson Miles Black compares railway signal roundels, the coloured lenses used on lanterns and signals, for how well they can be seen and distinguished. Its pictures are plain line diagrams of concentric rings labelled with corning percentages, a rotated chart of test curves, and a photograph of a railway lantern with a round lens, useful to a designer for simple concentric ring motifs and vintage signal hardware.
