Reading — the celestial arc the micrometer reports: the star's sideways drift on the wire plate divided by that telescope's own scale. Airy's 20.55″ is a reading, not a direct angle.

Internal angle (θ_int) — the physical tilt the ray must take inside the tube to produce that reading. It is implied, not measured; a 20.55″ reading needs 15.45″ inside the water.

Aberration — the incoming tilt θ_ext = arctan(v/c) = 20.55″, set by the observer's velocity. The reading recovers it.

Micrometer × n — the water plate is calibrated n times finer than air (Airy p.39: 27.8-inch air-equivalent vs 37.0-inch tube), so 15.45″ inside reads the same 20.55″ as air. That × n scaling is the whole reason the water null holds.