What phenomenon was observed by Sir John F.W. Herschel in 1839 and what type of photographic process did this?

From 1834 to 1838 Herschel discovered over 1,200 double stars and an even larger number of star clusters and hazy objects called nebulae He also charted the Magellanic Clouds, two galaxies visible in the Southern Hemisphere. His discovery that sodium thiosulfate dissolves silver salts contributed to the development of photography.