The photostat machine was an early 20th-century form of photo reproduction, which merged camera, printer, and darkroom into one large machine. It used a prism in front of a lens of an attached camera to create negative prints of documents and materials onto rolls of silver-halide and gelatin-emulsified sensitized paper, which were then cut off (either automatically using a cutter built into the machine or manually), developed, and fixed within attached trays, using a hood to simulate darkroom conditions, thus creating a photonegative which could be washed, dried, and photographed again for a positive print. Photostats were large, weighing approximately 750 lbs and measuring 7ft tall and 12 ft 6 in. long when the developing and fixing trays attached.

A figure of a photostat machine in 1934 with an attached board for placing objects.
The camera is labeled as item #41, and the developing and fixing trays are items 11 and 39 respectively.
Box 17 holds the roll of sensitized paper. Courtesy of a 1934 photostat instruction manual (linked below).

The photostat process was relatively simple and, compared to a traditional camera of the time, fast, requiring about twenty minutes per print (most of the time was the spent fixing and washing the print), and then that amount of time again to make a positive print from the initial negative.

First, the objects to be copied were placed on either the board (pictured) or, for smaller items, on a similarly-attachable booktray, and then were illuminated and then exposed to the camera for about eight seconds. Then, the sensitized paper with the undeveloped print was cut (either automatically or manually) and then printed directly into the developing tray, and then slid into the fixing tray once development was completed. Finally, the print was washed for about fifteen minutes and then dried, and then it was ready for use as a photonegative.

A photostat and its operator in Winnipeg in 1954, courtesy of the Folger Shakespeare Library.

Below are some additional resources pertaining to the photostat machine and some contemporary technologies.

The Photostat and its operation

A digitized 1934 instruction manual for a standard photostat machine, with more detailed operational instructions and process mapping.

The Photostat in reference work by Charles F. McCombs

A 1920 bulletin from the New York Public Library discussing some specific uses of the photostat in library contexts, with some helpful dates.

The Photostat in Bibliographical and Research Work: A Symposium by George Watson Cole 

A 1921 collection of bibliographer's perspectives on the merits, and drawbacks, of photostat technology.

Photographic Copying and Reproducing by Lodewyk Bendikson

A 1921 article outlining how photostats make early modern works accessible to libraries that do not have have access to original manuscript copies.