Immediately in front of each of these cameras, and detached therefrom, was placed an electro-photographic exposor, a side section of which is represented by Figure 2, in which A is a continuous band of thin rubber cloth impervious to light; the edges of which are bound with strong tape, and arranged to run in a groove, and over two rollers RR which are attached to a frame. In this endless band are two apertures OO of suitable size, and so arranged that their full openings as they pass each other shall simultaneously take place in front of the center of the lens L. The upper and lower edges of these apertures are kept taut by light steel rods attached to the tape binding. To the lower rod of the front aperture is fastened a ring C and a cleat, to which some elastic rubber bands B are attached; these bands are easily removable and their number increased at discretion; in some instances of rapid exposures a tension of twenty-five pounds or more was required. On a shelf of the frame is a magnet M, over the top of which is arranged a steel lever G pivoted near the end D which terminates with a slightly indented projection. [Illustration: Fig. 2.] The armature of the magnet is pivoted at H; its upper arm terminates with a shoulder I. S is a spring to prevent the accidental shifting of the shoulder from its contact with the lever when the exposor is ready for its function. N is a set screw to adjust the distance of the armature from the magnet. To prepare for a series of photographic exposures--the plates having been already placed in the cameras--the end of the lever G is placed under the shoulder I; the endless curtain is revolved until the front aperture O is raised to its proper position, when the ring C is hooked upon the projecting point D. A cord attached to the rubber bands B is drawn around the pulley P, and a ring at its end is slipped over a pin, which keeps the spring at a proper state of tension. Upon the completion of an electric circuit the armature is drawn towards the magnet; the end of the lever is released from its contact with the shoulder; the ring C is released from the projecting point D; the front of the endless curtain is drawn rapidly downward; the apertures meet in the center of the lens, form a gradually expanding and then contracting diaphragm, and the exposure is made. A front view of three electro-photographic exposors is seen in Figure 3. The first of these represents the exposor set and ready for an exposure; the second shows the meeting of the apertures at the commencement of an exposure; the third, their position near the completion of the exposure, they having in the meanwhile uncovered the lens to their full capacity. [Illustration: Fig. 3.] Figure 4 illustrates a portable battery of twelve electro-photographic exposors; it consists of a rectangular box divided into compartments, open at the front and rear. [Illustration: Fig. 4.]