The online archive of the Library of Congress has a huge digitized library of over 6,000 photochromic prints.

This printing technique produces images with surreal colors, kind of similar to colorized black and white photographs.

Most of the pictures were taken between 1890 and 1900, more than 120 years ago. The following pictures show Heidelberg and Mannheim.

Heidelberg, Blick vom Philosophenweg
Heidelberg, view from Philosophenweg
Blick von der Hirschgasse
View from Hirschgasse
Mannheim, Kurpfalzbrücke (damals noch Friedrichsbrücke)
Mannheim, Friedrichsbrücke (today: Kurpfalzbrücke)
Rheinbrücke (today: Konrad-Adenauer-Brücke)