Christiaan Huygens observed little animals using dark-ground illumination

March 14, 1678

Huygens wrote in Latin:

Rursus de supernatante pellicula inter vitreos orbiculos inclusi. ubi jam prodigiosa multitudo erat talium animalculorum, ac fi acervatim congesta forent, sed tantum exiguo motu, etsi satis celeri, inter se movebantur, qui motus ex splendore ipsorum percipiebatur, quia certo positu lucebant alijs vero minus. directe ad lumen obvertenti nihil fere de motu apparebat, sed luce a latere incidente.

English translation (via Google):

Again, the floating film is enclosed between the glass spheres where there was already a prodigious multitude of such animals, and they would have been heaped together, but they moved among themselves only with a slight movement, though quite rapid, which movement was perceived by their brightness, because in a certain position they shone, but in others less so. Turning directly towards the light, almost nothing of the movement was apparent, but you do see it when the light falls from the side.