By mechanical means a millimetre has been divided on a plate of glass into a thousand equal parts. There are animalculæ so small that their whole body placed between two of these divisions does not touch them. These living beings then measure at the most 1/1,000 of a millimetre, or what is nowadays called a micron. They have members, organs, muscles, nerves, etc. These organs are composed of cells and the cells of molecules. If the latter were only a hundredth part of the dimensions of the body (they are probably much smaller), these molecules would measure, if separated by intervals equal to their own size, 1/200,000 of a millimetre as before. Molecules or atoms, whatever may be the name applied to the ultimate particles of matter, are declared by present-day science to be equal to stellar systems or microcosms. * * * * * This modern teaching of microscopy was anticipated for a long time by thinkers. In his _Commercium Philosophicum_, published in 1745, Bernoulli wrote to Leibnitz concerning the imaginary inhabitants of a grain of pepper. “If these animalculæ had an intelligent mind and were capable of reasoning, they could flatter themselves that they and the drop of liquid which they inhabit constituted the entire universe. Imagine that a small grain of pepper in which under the power of the microscope we discover a million animalculæ had all its parts proportional to the corresponding parts of our world, that is to say, its sun, its fixed stars, its planets with their satellites, its earth, and its mountains, its fields, its forests, its rocks, its rivers, its lakes, its seas, and its diverse animals, can one suppose that the inhabitants of the grain of pepper, these _pipericols_, who would perceive all objects under the same visual angle and consequently in the same size as we do, would not believe that outside their grain nothing exists, and would have the same right to believe it as we believe that our world includes everything? For, I ask you, what reason or what experience would they have which would convince them of the contrary or would show to these poor little animals that there is another world incomparably greater than their own with inhabitants incomparably greater than themselves?” Therefore, concludes Bernoulli, if these _pipericols_ cannot know that, who is there among us who knows that this whole visible world is not perhaps a grain in comparison with another world incomparably greater?