I'm looking for a less clumsy way to say:
The phase advance in the measurement plane was varied between the corrector lenses and the measurement screen.
Also with the additional constraint that the variation has to be equally spaced which is defined in the next sentence. I would prefer to put all the information into one sentence. The context is a scientific paper.
The example sentence, as it stands, is ambiguous; on the one hand, it might be saying that phase advance was experimentally controlled; on the other, that measurements showed variation. Use an active rather than passive construction to avoid that ambiguity:
We varied phase advance in the measurement plane linearly between the corrector lenses and the measurement screen.
We measured linear variation of phase advance in the measurement plane between the corrector lenses and the measurement screen.
I've used the word linear (in sense 5, “Of or relating to a class of polynomial of the form y = ax + b”) to express one possible interpretation of the phrase “the variation has to be equally spaced”. Also consider use of proportional (“At a constant ratio (to), In proportion (to)”).
Building on @jwpat7's answer, I suggest the following to communicate that you used equally sized steps (e.g., 7):
We varied phase advance in the measurement plane in 7 equal steps between the corrector lenses and the measurement screen.
