Photostats revolutionized the study of rare books in the 1910s and 1920s. For the first time, libraries could quickly make reasonably affordable reproductions of their holdings available for consultation off site. Research that had once been cost-prohibitive because it required in-person travel or conventional photography became possible. Photostat machines exposed the image directly onto a long roll of light-sensitive paper, which was then developed inside the machine while new images continued to be captured: no need for a separate step with film negatives. Because photostats are direct images, the blacks and whites are reversed, like a film negative in traditional photography. Positive photostats are actually photostats of a negative photostat, not of the original text.
The Folger shelves photostats and photocopies of complete publications in the PR1400 sequence on the Reading Room balcony. Basically, any facsimile that does not have editorial apparatus goes to the PR1400s. Facsimiles with editorial content are shelved with the rest of the published books in the open stacks (or the vault, for extremely expensive facsimiles). Photostats and photocopies that have been added to the collection are cataloged in Hamnet, mostly as separate titles. The exception is the collection of 366 photocopied titles used by Edward A. Langhans in his promptbook research, which is divided into four collection-level records.
Photostats and photocopies on the Reading Room balcony are shelved by type of material:
- PR1400: STCs (with STC number)
- PR1401: Wings (with Wing number)
- PR1402: 18th century English publications (with number representing author or title)
- PR1403: 19th century English publications (with number representing author or title)
- PR1404: 20th century English publications (with number representing author or title)
- PR1405: Manuscripts, prints, and other non-book material (with number representing author or title)
- PR1406: Continental publications (with number representing author or title)
- PR1407.P7: Collection of photocopies assembled by Edward A. Langhans for his promptbook research (official documentation for this classification has not yet been found)
Original plans for the Folger Shakespeare Library called for a No. 2 Photostat machine, but in 1931 the plans were changed to accommodate a higher-capacity No. 4 Photostat machine. It was housed on the 2nd floor, in a two-room suite next to the elevator that has since been converted into a three-room office suite.
- "Photostats, or, The more things change, the more they stay the same," The Collation, July 23, 2015.