Fig. 4. Foucault's method for determining vergence of rays emerging from a retinoscope. A card or your hand is introduced close to the retinoscope and moved at right angles to the emerging rays. Observe the shadow produced in the unfocused image on a screen or wall in a darkened room. A. Rays converging at a focal point before screen cause an “against” motion. B. Rays converging beyond screen cause a “with” motion. C. Diverging rays cause a “with” motion. (Weinstock SM, Wirtschafter JD: A Decision-Oriented Manual of Retinoscopy. Springfield, IL: Charles C Thomas, 1976.)