Source photographs

How these were found

Searching Wikimedia Commons for a usable historical view is harder than it looks. These five rules were learned the hard way, one mistake at a time. They are set down here because nobody else seems to have written them.

1. Free-text search is capricious. “Pisa Italy Photochrom” returns ten files; “Siena Italy Photochrom” returns none — though both cities were photographed just as often. An empty result proves nothing about the collection.
2. The category name cannot be guessed. It is neither “Photochrom prints of Italy” nor “Photochrome prints of Siena” — both empty — but “Siena on photochrome prints”. The way to find the real name is to take a file you already have, read its categories, and walk the tree from there. That is deterministic; search is not.
3. Read three licence fields, not one. A public-domain file often carries no LicenseShortName at all. Read License and UsageTerms as well, or you will discard usable pieces by mistake.
4. Beware a width twice the height. In “Historical images of…” collections, a piece with a ratio near 2:1 is very often a stereograph — two nearly identical halves side by side. It looks like a fine wide panorama in a list of results, and it is not one.
5. Dimensions alone do not identify a piece. Three files in the same Library of Congress collection share the exact size 3488 × 2592. Picking one on that basis would have been wrong: only comparing the images themselves settled it — a mean difference of 3.4 out of 255 for the right one, above 41 for the other two.
6. To find which file an image came from, search by exact dimensions. Commons accepts filew: and fileh: as search operators — filew:7342 fileh:5466 returned three files where a keyword search across 175 candidates had returned none. Then confirm on the pixels, never on the title: resize both to a common size and compare. A mean difference below 3 out of 255 is compression noise, so the same image; a different photograph of the same monument gives above 70.

What a photochrom is

Most of the prints listed above are photochroms. A photochrom is not a tinted photograph: it is a colour lithograph pulled from a black-and-white negative, each colour asking for its own stone — a dozen or more for a single view. The process was developed at Zurich in the 1880s and worked by Photoglob Zürich, and under licence in the United States by the Detroit Publishing Company; those are the two names that recur in the table. The flat, saturated areas of colour are a consequence of the method, not a style, and they are what makes these prints so usable as a model for drawing. Their public-domain status is the one recorded by the collections that hold them, given in the table above — it is their statement, not our assessment.