This image was acquired from
flickr. It was marked as Public Domain or CC0 and is free to use. To verify, go to the source and check the information there.
Keywords from Image Description:
charles howard hinton charleshowardhinton the fourth dimension thefourthdimension geometric geometry illustration four dimensions fourdimensions mathematics physics science public domain publicdomain text writing indoor document Public Domain Source The Public Domain Learn more about Charles Howard Hinton and Tesseracts on Public