Before dry plate Photography (which was like film photography but the emulsion was coated on glass), there was wet plate photography. Wet plate photography required the photographer to coat a glass plate with emulsion, make a photograph, and develop the picture before the emulsion dried. If you wanted to take pictures, you had to take a darkroom with you. When dry plates came out, you could wait till you got home to develop them.

Then along came one George Eastman, who invented flexible film. Film is much more convenient than glass plates - it's lighter and it won't break if you drop it - so it rightly should have killed the dry plate. And for most purposes it did - but NOT for pro astronomy! Big telescopes mount their cameras so the film is laying flat, and they get left outside in the cold all night to image the night sky. Film sags in the middle in these cameras and it expands and contracts with temperature. Plates are better for making astrophotographs than film is, so until the observatories went digital Kodak maintained a plate coating line, and once a year they'd fire it up and make enough plates to fill the needs of science.

Before dry plate photography (which was like film photography but the emulsion was coated on glass), there was wet plate photography. Wet plate photography required the photographer to coat a glass plate with emulsion, make a photograph, and develop the picture before the emulsion dried. If you wanted to take pictures, you had to take a darkroom with you. When dry plates came out, you could wait till you got home to develop them.

Then along came one George Eastman, who invented flexible film. Film is much more convenient than glass plates - it's lighter and it won't break if you drop it - so it rightly should have killed the dry plate. And for most purposes it did - but NOT for pro astronomy! Big telescopes mount their cameras so the film is laying flat, and they get left outside in the cold all night to image the night sky. Film sags in the middle in these cameras and it expands and contracts with temperature. Plates are better for making astrophotographs than film is, so until the observatories went digital Kodak maintained a plate coating line, and once a year they'd fire it up and make enough plates to fill the needs of science.