Compton went to work in St. Louis and made an important modification to his plans. He used his Bragg spectrometer to select a single sector of X-ray wavelengths, as Stuewer notes in his book on the Compton Effect (p.163). This meant that he was working with X-rays of known energy and wavelength. To venture again into explaining what was going on in terms of what we now know (a century later), to get a clear view of what is happening with the Compton Effect and the conservation of energy, you have to know the initial wavelength of the light quantum (X-ray) that hits an electron and then the wavelengths and out-going directions of the resulting quanta. Of course, in 1921, Compton was still thinking in terms of waves or pulses of X-rays, as was everyone else except Einstein (but we’ll get back to Einstein on light quanta after following Compton’s elucidation of X-ray’s hitting electrons).
As Stuewer describes things in 1921, Compton agreed with some other experimentalists that the variations in X-ray scattering were the result of multiple events with multiple electrons (beta-rays) with Doppler-shifted diffraction happening at relativistic speeds. The observed longer (less energetic) wavelengths were due to secondary excitations in the target materials. On the other hand, Compton was able to measure how a change in the direction of what he considered to be the diffracted X-rays was related to the energy (wavelength) shift. He was driven to do this on October 10-12, 1921 in response to a paper by S. J. Plimpton that concluded that a mica crystal bent into an equiangular spiral did not diffract X-rays differently at different angles.
Compton set up his salt crystal (and paraffin), dosed it with X-rays of a single wavelength, and found that measurements at different angles of the resulting X-rays showed that the angle of the exiting X-rays was related to the observed energy shift. Which meant (as Stuewer points out) to Compton at that point, that his Doppler shifted secondary excitations were happening as he thought they were even though he had actually just observed and calculated something approximating what would eventually be known as the Compton Effect.
Well…he’s getting closer and here he is about 10 years later working on Cosmic Rays:
