This was a very inaccurate way of measuring a patient's haemoglobin. It was popular until the early 1950's, particularly in Europe and the USA, because it seemed quick and easy. A drop of blood was mixed with hydrochloric acid and converted to acid haematin. The mixture was diluted until it seemed by eye to match a coloured glass standard. It was later found that results were often up to 20% out, usually high, compared to subsequent methods.