I admit to being a bit of a Google books junkie when it comes to 19th-century stuff. This old, hard-to-find, well-out-of-print material was often the thankless task of the literary sleuth who just wanted to find (for example) as many 19th-century references as possible to one obscure 18th-century author. If the name is unique enough, Google books can bring up a lot of references.
It can also bring up a lot of fingers. Turns out that people scanning Google books have also reasonably often inadvertently scanned their fingers or whole hands, sometimes in color. I wish I was the first to notice this, but as soon as it occurred to me to blog about it I checked for "google books finger" and came up with plenty of hits. Others have noticed. Others have laughed.