Scaling Factors That Could Potentially Affect Huggins' Hydrogen Lines

  Relative to the minimum deflection wavelength for Huggins' spectroscope there appears to be a progressive shift to higher wavenumbers which is a blue shift. But relative to the limit of the series, λ_∞, there is a redshift. There are a number of factors which might cause a scale change in the observed wavelengths and need to be taken into consideration.

Calibration of the Spectroscope
minimum deflection
calibration curve

Air Wavelengths vs Vacuum Wavelengths
Definition of standard conditions
Snell's Law (measurement in air results in a blue shift relative to a vacuum )
Rydberg Constant (R_∞ is for a vacuum)

Light nucleus
R_M needed for light nucleus (R_∞ is for a heavy nucleus)
reduced mass
electron mass

Doppler shift

Gravitational redshift

All of the above contribute to a change in scale. What did Huggins miss?