Light boxes have been installed in shop windows in Lewes showing photographs of the town dating back to the 1860s.
The images have come from a photography shop which has been run by the Reeves family in the Sussex town since 1858.
Reeves Studio has an archive of more than 100,000 glass plates - some now shown in light boxes in 55 shop windows in the town.
The exhibition, Stories Seen Through a Glass Plate, is being held as part of Brighton Photo Biennial.
The current owner of the studio, Tom Reeves, great-grandson of Edward Reeves who opened the business, said the shop had been handed from father to son and father to son "all the way down".
He said the shop had kept all its negatives and also the business ledgers that went with them, forming the Edward Reeves Archive, which will now be catalogued and made more accessible.
One of the curators of the exhibition, Brigitte Lardinois, said experts were absolutely convinced of the archive's international importance.
Ms Lardinois, deputy director of the photography and archive research centre at the University of the Arts London, said: "A lot of these archives went down in the nineties when digital came. People have kept hold of plates, but they very often threw the ledgers away.
"The amazing thing about these archives is we have account books and negatives and letters, and it's an absolute treasure trove."
Out of the thousands of plates examined so far, half were portraits of Lewes people and the others were images of the town, including the Sussex Downs and people going to war, she said.
The exhibition, which runs until 2 November, includes pictures taken by three generations of the Reeves family and are shown in locations where they were originally taken.