We will now consider in detail the important case in which uniform plane waves are resolved at a surface coincident with a wave-front (OQ).
They are here focused (so far as the rays in the primary plane are concerned) upon the circle OQ' A, and the outstanding aberration is of the fourth order.
The grating at A and the eye-piece at 0 are rigidly attached to a bar AO, whose ends rest on carriages, moving on rails OQ, AQ at right angles to each other.