cathode-ray and canal ray tubes, a fluorescent lamp. Equipment to study the properties of light (and radio waves in general), including the wave–particle duality (most 1900–1910): a slit, a polarizer, a photo-electric cell, a Fresnel double mirror, lenses, an induction coil, a photosensitive resistor, a spectroscope, an oscillograph, a wireless receiver, a microphone, standards of self-induction. Also Zeeman's Nobel Prize diploma and medal and a lab diary, an electromagnet with polar pieces, 3D plaster models to visualize thermodynamic properties (e.g. phase transitions). 22 Medicine: Man as a machine Pieces at display include: a gigantic microtome, the first heart-lung machine (prototype) by inventor Jacob Jongbloed (1948), the first kidney dialysis machine by inventor Willem Johan Kolff, X-ray equipment, an iron lung, galvanometer tools, a bow and arrow used to draw ultra thin quartz fibres (by shooting away a droplet of molten quartz), the original golden Nobel Prize medal from Willem Einthoven (for developing the first practical electrocardiogram in 1903) 23 Astronomy: Celestial Accounting 24 Microscopy: New ways The room houses two electron microscopes (1947–1950) and related equipment (gold vaporiser, lens, object holders, a cooler, a goniometer), a collection of compound and specimen microscopes (some binocular) (1840–1870) plus accompanying microscope lamps and a tool box, a binocular polarisation microscope (1850), several microtomes (a.o. a cryomicrotome) and ultramicrotomes, a phase contrast microscope, a fluorescence microscope (plus several epi-illuminators, to exploit the difference between emitted and reflected light frequencies that some substances display), compound microscope with transformer in camera installation, a photomicroscope (photo camera with inverted microscope), a boxed collection of microscopic specimens (1870). The museum also contains a fountain pen that was gifted in 1921 by Albert Einstein to Paul Ehrenfest, then the director of the physics faculty of Leiden University. This pen was gifted to Ehrenfest as a thank you gift for the friendship that he and Einstein shared, and because Ehrenfest created a professorship for Einstein. Einstein was a 'bijzonder hoogleraar', meaning a professor by special appointment made possible by the Leids Universiteits Fonds from 1920 up to officially 1946. In the first half of his appointment he visited Leiden regularly to give lectures. There is a note with the pen on which is written (In German): 'This pen has been used for years by Einstein, especially during the time between 1912 and 1921 – So that all his designs and calculations relating to general relativity and gravity from this time