Sir John Frederick William Herschel (March 7, 1792 - May 11, 1871) was a notable English scientist and astronomer in the 19th century. He was a close friend of William Henry Fox Talbot. Talbot is credited with the development of the photographic process that led to modern photography. What he called photogenic drawing used a paper negative similar to modern film negatives. Talbot and his contemporary Louis Jacques Mande Daguerre (inventor of the daguerreotype) used a solution of table salt to stop sunlight from further developing the photogenic print.
In 1819 Herschel, also interested in the development of this new art and science, developed a process for stopping the development of the image using a hyposulphite of soda. Hypo is the term still used for this purpose. However, the solution has been modified and is now sodium thiosulphate. The hypo process was introduced to Talbot and became the primary method of completely removing unexposed silver salts from the print. It was also Herschel who introduced the chemical gallic acid to photography. This photosensitive chemical is what led to Talbot's Calotype.
Talbot was fortunate that Herschel was much less secretive about his discoveries then Talbot was himself. Herschel was never able to reach a point where he could patent his processes but his suggestions greatly furthered the work of Talbot.
Herschel continued to conduct his own experiments concerning the photogenic process. He researched the effectiveness of different chemicals and silver halides. While experimenting he discovered the use of ferric salts to create what are known as cyanotypes or blueprints.
In addition to contributing chemicals process and discoveries to the art and science of creating permanent images with light, he coined many terms. He suggested to Talbot that he begin to call his process photography rather than photogenic drawing. Photography, meaning light writing, was a broader term that could fully encompass the process. It is thought that this term was first used by Hercules Florence and astronomer Johann H. von Maedler.
Talbot’s process used paper negatives that were used to produce a final positive image. It was in a paper presented to the Royal Society in 1839 entitled, "Note on the art of Photography, or The Application of the Chemical Rays of Light to the Purpose of Pictorial Representation," that Herschel first coined the terms negative and positive to describe the process. These too are terms that are used in modern photography. A fourth term that Herschel is credited for is the snap-shot.
Sources:
“A World History of Photography, third edition” Naomi Rosenblum
“A History of Photography” http://www.rleggat.com/photohistory/history/herschel.htm