" When a Stop is placed between the combinations of a double lens, the effective aperture is not given by measuring its diameter. Rays of light, directly they pass through the outside lens, converge, and evidently more light will pass through the stop than it would do were it in front of the lens, as it is in a single lens. The proper way to ascertain the effective aperture of a stop in this instance is to focus some distant object, and to place in the position of the ground glass a piece of card with a small hole pierced in the centre. If a candle be brought near the hole, a circle of light will illuminate the front lens. The diameter of this circle of light which illuminates the lens will be a measure from which the effective aperture can be calculated. Every stop should be placed in position, and the diameter of the illuminated circles measured and noted." - Sir W. Abney. "Instructions in Photography." The diameters of the illuminated circles of light divided into the distance between the stop and the focussing screen, will give the focal values or f numbers.