What did roll film replace?

Roll film replaced camera-speed roll paper. George Eastman's original Kodak came to the user loaded with a roll of camera-speed paper (as opposed to enlarging-speed paper, camera-speed paper was usable to take pictures on). You shot the 100 images on the roll and sent the camera, along with one dollar, to the Kodak Labs in Rochester, New York. There they would develop the paper roll, strip the emulsion from it and bond it to a strip of celluloid for printing.

What replaced glass plates was sheet film, for all uses except astronomy. Astronomers continued to use plates because they're more dimensionally stable than film, and they don't sag when you put them in a camera that's tilted back at a severe angle.

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Roll film replaced individual photographic plates, which had already progressed from coated glass plates to emulsion-coated stock. Glass plates were still popular for their comparative clarity. This "film" had a light-sensitive material in a thin layer attached to cardboard, which was developed and transferred to glass for printing.