Sodium Thiosulphate or "Hypo"

Sir John Herschel (1792-1871)

Englishman Sir John Herschel discovered a chemical for fixing photographs that revolutionized photography. His sodium thiosulphate (or "hypo") is still in use today.

Herschel was the son of William Herschel, the most famous astronomer of his day and the man who discovered Uranus. Of all the contributors to the invention of photography, John Herschel had the deepest understanding of light.

Herschel had discovered the dissolving effect of sodium thiosulphate on silver salts in the 1820s. In 1839, his fellow Englishman, William Henry Fox Talbot, and France's Louis Jacques Mandé Daguerre were experimenting with capturing camera images using light. Partly motivated by the sketchy details of their research, Herschel applied his knowledge to the same task. He discovered that paper sensitized with silver salts, exposed to light in the camera, and fixed with sodium thiosulphate resulted in a permanent image.

Herschel shared his discovery with Talbot, who published it with his permission. Both Talbot and Daguerre adopted it immediately in their own processes, as did almost all of their successors.

Herschel invented the word "photograph," from the Greek photos (light) and graphos (to draw). He also coined the photographic terms "negative," "positive," and "snap-shot."