Around 1916, Michael Woolf, an individual from New York City, purchased a phoropter (an instrument used to measure refractive error) and added his own innovation: a set of cylinder lenses ranging from 0.25 to 2.00 D, along with Risley prisms. This device was also known as the Ski-Optometer and weighed approximately 3 lb. 13 oz. In 1924, the patents and rights were transferred to the General Optical Company, which then produced a larger, heavier instrument called the Genothalmic Refractor, incorporating Woolf's 1917 patent H: 12" W: 10" D: 4"