_Apparatus and Material_ used in micro-photography have, from time to time, been greatly varied by different workers, some preferring to use the microscope in the vertical position with the camera superimposed or fitted on the eye-piece of the microscope tube; others, again, prefer that both the microscope and the camera should be arrayed horizontally. In another form the ordinary microscope is dispensed with and the objective stage and mirror are adapted to the front of the camera, together with a suitable arrangement for holding the object. Lastly, the camera is lain aside, and an operating-room rendered impervious to light, takes its place, and the image is projected and focussed upon a ground glass screen held in its place by a separate support. This method has been made practical since the introduction into microscopy by Zeiss of the _projection_ eye-piece. It is well known that micro-photographs can be produced by employing these projection eye-pieces, as well as for screen illustrations in the lecture-room. [Illustration: Fig. 172.--Swift’s Horizontal Apparatus.] With regard to the position of the microscope and camera, the horizontal affords greater stability than the vertical, and is on this account to be preferred. The simplest apparatus consists of a camera fixed upon a base board, four or five feet in length, upon which the microscope can be clamped, and which also carries the lamp and bull’s-eye lens (Fig. 172). This arrangement I have found economical and useful. No more elaborate arrangement is actually necessary. Sunlight is no doubt the best, but a good paraffin lamp is a handy and available illuminant. With the former, and rapid plates, a short exposure of three or four seconds, even when high powers are used, is found sufficient; whereas, with the paraffin lamp it will vary from three to ten minutes. Walmsley gives the following table for exposures with the lamp:-- 1-1/2-inch objective 3 to 45 seconds. 2/3-inch " 7 to 90 " 4/10-inch " 1/2 to 3 minutes. 1/5-inch " 2 to 7 " 1/10-inch " 4 to 10 " For micro-photography the following practical rules must be observed. The sub-stage condenser may be dispensed with when low powers are used, as well as the mirror, and the lamp so placed that the image of the flat of the flame appears accurately adjusted in the centre of the field of the microscope. The bull’s-eye lens is so interposed, that the image of the flame disappears, and the whole field becomes equally illuminated with high powers; the sub-stage achromatic condenser must be used, and a greater intensity of illumination is obtained by placing the lamp-flame edgeways. It is advisable to begin the practice of micro-photography with low powers, and a trial experiment should be made with some well-known object as the blow-fly’s tongue.