This image was acquired from
wikimedia. It was marked as Public Domain or CC0 and is free to use. To verify, go to the source and check the information there.
Helmholtz Das Telestereoskop. Das Telestereoskop In Annalen der Physik und Chemie Band Joh Ambr Barth Leipzig image on ub uniheidelberg de UB Heidelberg Hermann von Helmholtz PDold Hermann von Helmholtz
Page 2023 Free-images.com. All images are Public Domain