Fig. 5. Top. Basic design of the Macbeth illuminometer. (E, eye position; D, comparison scale; K, rack and pinion; R, calibrated scale; T, test plate, S, standard lamp). Bottom. Schematic diagram of a Salford Electric Instrument Company photometer. (Hardy AC, Perrin FH: The Principles of Optics. New York, McGraw-Hill, 1932)