Leonardo da Vinci, in his celebrated treatise on painting, in speaking of the difference between laughing and weeping, says: 'In eyes, mouth, and cheeks there is no difference between one who laughs and one who weeps. They are distinguished from each other only by the position of the eyebrows, which contract during weeping, while in one who laughs they are drawn upwards. He who weeps raises the eyebrows at the inner corners, draws them together, wrinkling the skin above them, and turns the corners of the mouth down, while one who laughs draws the corners of the mouth upwards, and has an open, uncontracted brow.’ With these words da Vinci shows the characteristic expression of the face in laughter and in tears; but the physiologist is not content with what satisfies the artist, he seeks the cause and origin of the phenomena, analyses the reason of the difference and similarity in the expression of laughing and weeping, joy and pain. The impetus which Herbert Spencer and Charles Darwin gave to the study of human nature was so powerful, the progress made so rapid, and the new horizon so vast, that numerous philosophical and scientific books at once became obsolete; and when we now turn over their leaves, we seem to feel the breath of decay, as though we were groping amid the ruins and dust of edifices that have crumbled for centuries. The study of nature was much easier for the spiritualists and philosophers of the old school than for us, because with little trouble they found reasons which satisfied them, and in their faith they had a strong bulwark that sheltered them from the uncertainty and doubt which follow us everywhere as we grope deeper and deeper, trying to find out the causes of things. Duchenne de Boulogne, in his well-known book on the mechanism of human physiology, printed in 1862, still maintains that the facial muscles were created for the expression of the soul. 'Le créateur n’a donc pas eu à se préoccuper ici des besoins de la mécanique; il a pu, selon sa sagesse, ou--que l’on me pardonne cette manière de parler--par une divine fantaisie, mettre en action tel ou tel muscle, un seul ou plusieurs muscles à la fois, lorsqu’il a voulu que les signes caractéristiques des passions, même les plus fugaces, fussent écrits passagèrement sur la face de l’homme. Ce langage de la physionomie une fois créé, il lui a suffi, pour le rendre universel et immuable, de donner à tout être humain la faculté instinctive d’ex primer toujours ses sentiments par la contraction des mêmes muscles. 'Il était certainement possible de doubler le nombre des signes expressifs de la physionomie; il fallait, pour cela, que chaque sentiment ne mît en jeu qu’un seul côté de la face. Mais on sent combien un tel langage eût été disgracieux.’[24]