If earlier types of gunsights as Revi was NOT gyroscopic (I assume that) was the whole device only for projecting a light reticle on the glass?. Why don´t just paint the reticle, if it had not movement. Thanks for explaining (or any link).
Two reasons - first, because the size of the aiming circle could (on many examples) be adjusted to match the wingspan of the target so the pilot would know when he was at the harmonisation range.
Second (and much more important) because the aiming mark remained on target regardless of the position of the pilot's head: if he moved his head too far out of line, the aiming mark would disappear off to one side. With a painted aiming mark, you would have to be certain that your head was in exactly the right position, otherwise the sights would be wrong.
Tony Williams: Military gun and ammunition website and discussion forum
The key idea is that a Collimator projects an image to the pilot's eye. His eyes are focused at infinity rather than at the instrument panel. Optics
Here is a description of common types of gun sights: FN FAL Rifle