The _Projection Eye-piece_ has become an essential part of micro-photography, and it is so arranged that it may be employed with advantage with objectives of either the apochromatic or ordinary series for photographic purposes, projecting an exquisitely sharp image of the object on the plate. A diaphragm between the lenses limits the field, and a sharp image of it should appear on the screen when the eye-piece is adjusted. The adjustment may be effected by revolving the eye-piece cap in a spiral slot, so that the eye or top lens is either brought closer or removed farther away from the diaphragm, as may be required, and divisions and a reader are usually provided for registering positions. Such eye-pieces are made to fit any size microscope body. Initial magnifying powers:-- English length of tube 10-in. 3 and 6. Continental " " 6-in. 2 and 4. [Illustration: Fig. 173.--Baker’s Pringle Vertical Micro-photographic Apparatus.] The microscope and camera (Fig. 173) are here seen to be part of the same instrument. The bellows of the camera have an extension varying from 6 in. to 30 in. The board on which the microscope and limelight jet are fixed is made to turn out of the line of the camera to facilitate adjusting the instrument and radiant, either limelight, electric light or paraffin lamp; when this is done the board carrying the same is turned back to a stop which brings the microscope into a central position with the focussing screen. An adjustment is supplied at the side of the camera, geared to the slow movement, for finely focussing the object upon the screen. A light-excluding connection is fitted to the front of the camera and microscope; immediately behind this, in the bellows, is an exposing shutter which is manipulated by means of a small milled head. Two focussing screens are usually supplied, one grey, and one patent plate, together with a double dark slide. Mr. Andrew Pringle’s vertical micro-photographic apparatus is an excellent form; it consists of a heavy base and brass support, carrying a quarter-plate camera, grey and plain glass focussing screen, double dark back, camera extending to 24 inches, and turning aside as shown in Fig 173. It is light-tight in all its connections. To secure uniform results in micro-photography, only thin preparations, which lie as nearly as possible in one plane, can be relied upon for good and perfect negatives. [Illustration: Fig. 174.--Ross’s Arc Lamp.]