Electrical Excitation of m. zygomaticus minor and m. corrugator supercilii: Painful Weeping, c. 1856, plate 49 from The Mechanism of Human Facial Expression, published 1862

In collaboration with photographer Adrien Tournachon, French neurologist Guillaume-Benjamin-Amand Duchenne de Boulogne recorded electrically induced facial expressions to illustrate his book on the subject. This print offers a rare example of an albumen print whose brown and purple tones still look as vivid as the day it was made. The rich color is an unintended byproduct of the addition of gold chloride (a chemical compound of gold and chlorine) to the processing bath, done to increase the stability of the image. Albumen images generally convey sharp details because the albumen binder prevents the silver particles from sinking into the fibers of the paper support. This sharpness is especially suitable for scientific images.