Petapixel:

If you come across any photograph published in the US before 1923, you’re free to use it for whatever purpose you’d like, with or without permission, and with or without attribution. Why? Because its copyright has expired and it’s public domain.

Strangely enough, sometimes free public domain photos get sold as stock photos, and those who don’t know any better may pay large sums to use something they could have used for free.

Don’t get conned by Corbis or anyone else trying to sell you imagery that is in the public domain.