Nicolaas Hartsoeker (26 March 1656 – 10 December 1725) was a Dutch mathematician and physicist who invented the screw-barrel simple microscope[1] c. 1694.

Nicolaas started to make a living as a lens maker in Rotterdam, and was instructed in optics by Antonie van Leeuwenhoek.

In June 1678, Hartsoeker accompanied, in the role of an assistant, Huygens on a trip to Paris, where they made a great impression with their microscopes.

For example, Hartsoeker disavows the contemporary position (e.g. of Robert Hooke) that with refractor telescopes one soon would be able to see man-sized creatures on the moon, if any in fact existed.

Later, in 1704, Hartsoeker accepted the offer of Johann Wilhelm, Elector Palatine to become "first mathematician and honorary professor of philosophy" at the University of Heidelberg.