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Johannes Stark measured the splitting of atomic spectral lines in a strong electric field. The Stark effect connects that applied field with changes in atomic energy levels and therefore the frequencies of emitted light. Resolving the splitting requires control of the electric field, the emitting region and the spectral resolution. Different observed components need not shift equally: the pattern tests how the electric perturbation changes the available quantum states, rather than merely showing that the light source is affected.^[ref][ref]

By Johannes Stark. Nobel lecture delivered 3 June 1920, associated with the 1919 prize; cited pp. 430–435. Inspected Columbia-hosted collected-volume reproduction.

Awarded to Johannes Stark for the Doppler effect in canal rays and the splitting of spectral lines in electric fields. The Stark canal-ray experiment follows the measurement.^[ref]

By Johannes Stark. Nature 92, 401 (1913). Publisher lists publication 1 December; issue date 4 December. Only the two publicly exposed report paragraphs were inspected.

In 1913 Johannes Stark reported that an applied electric field resolved the hydrogen $H_{\beta }$ and $H_{\gamma }$ spectral lines into five components at his instrument's spectral resolving power, viewed perpendicular to the electric field. Three inner components had polarization of an electromagnetic wave perpendicular to the electric field; two outer components had it parallel.^[ref]

In his lecture of 3 June 1920 for the 1919 prize, Johannes Stark describes canal rays emerging through a perforated cathode into the electric field between it and a supplementary electrode. A spectrograph recorded the emitted light. His earlier Doppler effect comparison used a collimator perpendicular to the beam and then along approaching rays. Later beam/electric field reversal changed component intensities.^[ref]

The historical account distinguishes observation perpendicular to the electric field, with five resolved components, from observation parallel to it, with three. These are viewing directions, distinct from the direction of the polarization of an electromagnetic wave.^[ref]

The experimental distinctions matter. Changing the viewing direction tests a Doppler effect contribution; changing the applied electric field tests the Stark effect; analyzing polarization of an electromagnetic wave separates components that may overlap in an unpolarized record. A comparison with the unperturbed spectral line fixes the origin of a displacement. These are different controls, and the earlier Doppler effect comparison should not be described as a field-off control of the later discovery. A spectrograph's finite spectral resolving power can merge several components into one. In particular, a count of visible spectral lines does not by itself count all energy eigenstates.

For a modern idealization of the gap, an electric potential difference $\Delta V$ over separation $d$ gives $|\mathcal{E}|\simeq |\Delta V|/d$ only if edge effects and the charge density in the gas are negligible. This is a calibration model, not a historical value assigned to Stark's apparatus. Similarly, a canal ray beam need not be a pure beam of neutral excited hydrogen atoms; the quantum mechanics model below isolates one neutral hydrogen atom from the source's preparation and collision processes.