This unusual camera, made by the English Thornton Pickard company,  was a replica, identical in size, shape and weight to an American Lewis Gun. It was used to train British World War I airmen in aerial combat. The 300mm f/8 lens was in the barrel with the film loaded into a chamber behind it. A screen was positioned in the chamber which, depending on its type, recorded markings on the film either as ruled lines dividing the screen into squares or as a series of concentric circles. As the camera was aimed at its target and the trigger pulled, a picture was taken with a single speed of around 1/150 second. When developed, this indicated how accurate the gunner had been in lining up his target. The camera also featured a fake bullet magazine. As this was removed and replaced, it pushed down on a button which connected with a device to perforate a small hole in the film. This proved that the gunner had not only aimed and fired the gun correctly, but that he had also changed the magazine at the appropriate time, in the way he would have to if operating the real thing.