License, verbatimPD
The National Gallery of Art: open access image with no usage restrictions under the NGA's open access policy (openaccess=1 in the Gallery's published_images dataset).
Guillaume-Benjamin-Amant Duchenne (de Boulogne). Plate 14: Designed to show a comparison between the maximal contraction of the superior part of m. orbicularis oculi and m. corrugator supercilii, in the same individual. On the left, very strong electrization of the superior part of m. orbicularis oculi: dissatisfaction, somber thoughts. On the right, electrization of m. corrugator supercilii., 1854-1856, printed 1862. National Gallery of Art, Washington.