Inventor of the optometer (1838-1890).Constant Loiseau was born in Namur, Belgium. He became a doctor like his father, and joined the Belgian Army Medical Corps as a specialist ophthalmologist. At this time, military service was compulsory, and many conscripts tried to avoid it by claiming their eyesight was too poor. The army doctors had no means of disproving these claims, and so Loiseau invented a scientific instrument to measure the refraction of light by the eye, which he called an optometer. This allowed the doctors to identify most of the common sight defects, and led to the development of optometrists (specialists in prescribing spectacles). Loiseau’s optometer was taken up all over the world and was in common use until 1918. He died of diabetes in Leuven.