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.
2
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.
3
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).