[lreqztkn] History of the size of an atom

When, and how, did we discover the rough size of an atom?  Assuming atoms are hard spheres, this is equivalent to measuring the spacing between atoms in a solid, e.g., a crystal lattice.  So the answer came from X-ray crystallography, though we need to know the wavelength of an X-ray to calibrate.

Knowing what we know now, we can do the following to measure the wavelength of X-rays.  Set up an X-ray tube of a known high voltage.  Assume we can measure voltages accurately.  We know the charge of electron (Millikan oil drop), so can deduce the maximum energy of X-rays being emitted (Charles Glover Barkla).  Make a leap of faith that the X-rays actually are near those energies.  Divide by h (measured by Planck) to go from energy to frequency, then apply the speed of light (Fizeau and Foucault, later Michelson) to get wavelength.  Demonstrate diffraction (Max von Laue, also Arnold Sommerfeld?) to show that inter-atomic distances are on the same scale as the wavelength of X-rays.

(There's a pile of important experiments and Nobel prizes there.)

How did the determination atomic scale actually historically happen?  Was it shocking when we first learned how small atoms are?

On the opposite end of scale: history of the cosmic distance ladder.  I think it was shocking when we first learned how far stars, then galaxies, are.