What is understood by instantaneousness? To our knowledge, no definition thereof has as yet been given. For our part, we propose to style "instantaneous" any photograph that is taken in a fraction of a second that our senses will not permit us to estimate. The shutter is the apparatus which allows the light to enter the photographic chamber during this very short time.
In order to examine the different rules that govern the question of shutters, we shall take as an example the type styled the "Guillotine."
This apparatus, as every one knows, is a stiff plate containing an aperture and passing over the line of the rays of light. Some place it in front and others behind, while others again place it within the objective. Let us examine and discuss what occurs in the three cases. Suppose a rectilinear objective of the kind most usually employed in instantaneous photography, and an object, A B, that we wish to reproduce (Fig. 1), the objective being provided with any sort of diaphragm. The point, A, sends a bundle of rays, a"b", to the first lens. Here they are slightly refracted, and then go on parallel lines to the second lens, where they are again refracted and form at A' an image of A. It is this image that we see upon the ground glass, and which makes an impression upon the sensitive film. The point, B, behaves in the same way and gives an image at B', but, as will be at once seen, the image will be reversed. In our figure, A corresponds to the sky and B to the earth. If, then, the shutter passes in front of the objective, it will first allow of the passage of the rays which come from the sky, then, on continuing its travel, it will unveil the landscape, and lastly the ground. As it is submitted to the law of the fall of bodies and has a uniformly increasing velocity, it follows that the time of exposure will uniformly decrease between A' and B', and that the sky will pose longer than the foreground. Such a result is contrary to all photographic rules, which require that objects shall pose so much the longer the less they are lighted. This position of the "guillotine" shutter is absolutely false, and must be altogether discarded. If the shutter be placed behind the objective, it will follow, as a consequence of the same demonstration, that the time of exposure will go diminishing from B' to A', and that the foreground will be exposed longer than the sky. The solution is logical, then, and will permit of obtaining excellent negatives.