Apologies if this is naive, I was looking for a free book for a flight a week ago and I downloaded 'The Einstein theory of relativity; a concise statement' by H

Apologies if this is naive, I was looking for a free book for a flight a week ago and I downloaded 'The Einstein theory of relativity; a concise statement' by H A Lorentz, 1920, available here https://archive.org/details/einsteintheoryr00einsgoog https://archive.org/details/einsteintheoryr00einsgoog as well. On page 22, Lorentz states a 3rd experimental test for the theory, "If his (Einstein) theory is correct as it stands, there ought, in a gravitational field, to be a displacement of the lines of the spectrum towards the red." He adds, "No such effect has been discovered. ... there is no way of accounting for this failure if Einstein's theory in its present form is assumed." He goes on to say that some modification would be necessary. What eventually happened to this 3rd test?