This is the original glass-plate negative of the iconic photo of construction workers eating lunch on a girder during the construction of Rockefeller Center's RCA building in 1932.
Today, those of us who remember using analog cameras remember taking photographs on film — literally a plastic (or, earlier, paper or celluloid) strip covered with a film of light-sensitive chemicals. But once upon a time, when you took a photograph, you took it on a piece of glass. The glass could be as small as 3 inches by 5 inches, or as big as 11" x 17". Originally, each piece of glass had to be dipped into an emulsion of chemicals on site, and the photograph taken before the chemicals had dried — a time on the order of minutes. That meant that, wherever photographers went, they had to carry an entire darkroom with them.
George Eastman is credited with inventing film in 1884, but the type of glass negative you see above was an important in-between step, linking the cumbersome wet-plate process and the infinitely more convenient world of film. Instead of a bath of chemicals, it used a gelatin — with chemicals suspended inside like the bits of pear in a jell-o salad. Gelatin glass plates didn't need a portable darkroom. And they could capture an incredible level of detail. According to the Dayton History Center, you can read the writing on tiny signs in the background of some of these shots.
The downside: Taking a photo with these plates required exposure times that would seem incredibly long to us today. For instance, a basic film photographer just working with a point and shoot camera might use a film speed of 400 ISO. According to an artist working with glass plate photography today, that process produces an equivalent "film speed" of about 1/2 an ISO — two whole seconds to get an exposure on a nice, sunny day outdoors. That doesn't sound long, but it can be an eternity if you're trying to capture a spontaneous moment, like one guy lighting another's cigarette.
This is part of why we know that the famous photo of construction workers on a girder was actually a staged shot. That ... and the fact that there are other much-more-clearly staged shots from the same sequence of photos. At the New York Times Lens blog, you can see those photographs, and learn more about the history of an iconic New York scene.