Casper Wesselmade a significant contribution to mathematics, but his legacy is to be a footnote in histories of other great mathematicians. Karl Friedrich Gauss and Jean Robert Argand are most often given credit for expressing complex numbersas geometric shapes, but it was an idea that Wessel, a surveyor and map-maker, was first to develop. Because he was not a professional mathematician and made no great effort to publish his one breakthrough paper, it went largely unknown for a century.

Wessel was born on June 8, 1745, in Jonsrud in Akershus county, Norway (then part of Denmark), to Jonas Wessel, a church vicar, and Maria Schumacher, his wife. He attended the Christiania Cathedral School in Oslo from 1757 to 1763 and then spent a year at the University of Copenhagen. Wessel's career in cartography, or map-making, began in 1764, when he became an assistant to the Danish Survey Commission soon after leaving the University...