Herapathite

Herapathite, or iodoquinine sulphate, is a chemical compound whose crystals are dichroic and thus can be used for polarizing light.

According to Edwin H. Land, it was discovered in 1852 by William Herapath, a doctor in Bristol. One of his pupils found that adding iodine to the urine of a dog that had been fed quinine produced unusual green crystals. Herapath noticed while studying the crystals under a microscope that they appeared to polarize light.

Herapathite's dichroic properties came to the attention of Sir David Brewster, and were later used by Land in 1929 to construct the first type of Polaroid sheet polarizer.