More background on it: <a href="http:&#x2F;&#x2F;www.nypl.org&#x2F;research&#x2F;collections&#x2F;digital-collections&#x2F;public-domain" rel="nofollow">http:&#x2F;&#x2F;www.nypl.org&#x2F;research&#x2F;collections&#x2F;digital-collections...</a><p>A visualization tool: <a href="http:&#x2F;&#x2F;publicdomain.nypl.org&#x2F;pd-visualization&#x2F;" rel="nofollow">http:&#x2F;&#x2F;publicdomain.nypl.org&#x2F;pd-visualization&#x2F;</a>