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 -.