The assumption has been tested by measuring red-shifts both with a grating spectrograph and a prism spectrograph.

2 Since E = constant = h / c, the above statement assumes that the relation between Planck's constant h

and the velocity of light c is essentially the same for light from the distant nebulae as in light from

laboratory sources. The assumption has been tested by measuring red-shifts both with a grating

spectrograph and a prism spectrograph. The fact that the shifts are the same in both cases seems to

confirm the constancy of h / c. (Annual Report of the Mount Wilson Observatory, 1935-6.)

https://ned.ipac.caltech.edu/level5/Sept04/Hubble/paper.pdf