A spring catch in the frame locates the plate holder in the correct position. There are two pairs of positions marked on the dark-slide: one is for two exposures 3 ¼" x 4 ¾" on a whole-plate; the other is probably for exposures on smaller individual plates held in a reducing frame in the plate holder and with a mask over the aperture in the frame. There is a clip on the frame possibly for this purpose.

The Simplex Slide and Reversible Back (the name varies slightly) was sold separately or as part of the Simplex camera. The manufacturer, Hopkins Bros., only advertised for a few years in the 1880s. The back was probably intended for use in studios, fairgrounds and similar outlets for Midget and small portraits where the speed of switching from composing to exposure was important.

The repeating back consists of a box containing a lever-mounted focusing screen with a slot at the top of the box to which a slide can be clipped. Ordinary glass plates are attached to each side of a frame, the frame is pushed into a slide just large enough to hold the frame, at the top edge of the slide is a hole into which a brass rod can be inserted where it screws to the frame. To make an exposure the image is composed on the focusing screen, the slide is clipped to the box, the plate carrying frame can now be pushed into the back by pushing down on the rod, after exposure the frame is drawn up by using the rod. The slide can be reversed to expose the plate on the other side of the frame.

There are three notches on the back of this example for horizontally aligning multi-exposures but there is no vertical movement to expose the other half of the plate, presumably the repeating back could, as the name suggests, be mounted upside down or reversed.

It seems that the lever arrangement for the focussing screen was a later improvement (described as New in advertisements) and that earlier the screen was removable. The Simplex back or camera was shown and described at several RPS meetings. J & A.G. Hopkins were also photographers with a studio in Hoddesdon.