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.
Dt \c3g.) 3qr Oq,~
8, we have O'Q'/OQ = a' tan w' la tan w = 1/N, where N is the " scale " or magnification of the image.
If Q lb be the weight of the poise Q, the position of Q when the steelyard is exactly in balance is given by the equation I T: X q.d Q X OQ, or OQ =p X q X T -.