Extract from PRACTICAL MICROSCOPY by George Davis, 1889:

Collins' " Harley " binocular  is now constructed on the Jackson model. The base is large and made in one piece, the limb carrying the body, stage, substage, and mirror is in one piece also, with a machine-planed groove from end to end. A rackwork of six inches in length is attached to the body so that very low power objectives can be used. The fine adjustment is placed between the tube and the limb. The stage is circular in form and with concentric rotation, with a simple arrangement for adjustment should it get out of the centre. It has a clear aperture underneath, of three inches, when the apparatus plate is removed, and consequently very oblique illumination can be obtained by means of the mirror alone.