Agnes Catlow's Drops of Water is an 1851 book filled with beautiful illustrations of the author's interpretations of microscopic organisms.
The illustrations are all so delicate and ethereal looking. Each one feels like a doorway into another world. It's a lot of fun to zoom into each microscopic organism and see the great detail of these drawings. The color palettes are wonderful, too.
From The Public Domain Review:
" Agnes Catlow's Drops of Water; Their Marvellous and Beautiful Inhabitants Displayed by The Microscope (1851), was published toward the beginning of miscroscopy's second wave. Focusing on animalcules, or Infusoria (the "little animals" that infuse stagnant water, undetectable to the naked eye), Catlow acts as a friendly guide to any amateur in possession of a good microscope — an inexpensive instrument that not only provides entertainment "at all seasons of the year" but permits us to enter an ocular wonderland."
Previously: Metropolitan Museum of Art makes more than 375,000 public domain images available as CC0