ANIMATED PHOTOGRAPHY We have already commented upon the difficulties of the instantaneous shutter as a means of representing rapidly moving objects, in that the results do not usually convey more than the idea of suddenly arrested motion. A snap shot of a stage in the Derby contest fails utterly in comparison with the old racing print. The former can include only one particular set of muscular contractions, while the latter con ventionally represents the successive series of movements involved when horse and rider are straining their utmost to be first at the winning post. And the very perfection of the modern multispeed or focal plane shutter, by its sharp delineation of detail, is an element in destroying the artistic effect.
In short, the great fault of the snapshot instantaneous photograph is that it gives us a faithful record of say 36 sec., whereas the event, witnessed by the observers as one con tinuous whole, occupied several seconds before it was com pleted. A single snapshot is but a small fraction of the story. But if we can take a sufficient number of these snapshots at short intervals, and then present them one by one to the spectators, a fair impression can be conveyed of what actually took place. The cinematograph does more than this. It causes the series of records to blend into what is apparently one photograph, in which the characters move and perform their parts at approximately the same rate, and in the same manner, as in real life.
The Optical System.—We need not dwell at great length on the mechanism of the cinematograph machine, which serves to project the pictures on the screen. The strip of film containing these pictures is drawn past the lens system of the projection lantern by a series of jerks, moving exactly one picture's length with each jerk. In the old Zoetrope, or " wheel of life," of our childhood, the pictures were viewed through a black cylinder pierced with slits at intervals : in the cinematograph a revolving shutter cuts off the light during the change of pictures, and so performs the same office as the perforated cylinder.
Persistence of Vision.—It is this well-known phenomenon that renders the " living pictures " possible. An image once formed on the retina of the eye is, so to speak, photographed there for the time being. Years ago this was demonstrated in the advertisements of a famous soap ; we were invited to gaze for a few seconds at the huge red letters on the hoard ing, and then to turn our eyes upwards to the sky, when it seemed as if the letters were printed there. The image remains on the retina for a length of time, ranging from to sec. So that if the pictures succeed each other at the rate of fifteen or twenty to the second, and the changing process occupies only a small fraction of the time, say 1- of the space of time during which each picture is visible, the successive pictures will be seen blended into one. Were they to be drawn at a continuous rate without the intervention of a shutter, the effect would be a mere blurring.
The Camera.—A similar piece of mechanism must be included in the instrument used for producing the pictures, so that each photograph may be taken, and the film jerked on for the next exposure in exactly the same way as it will afterwards be projected upon the screen. Otherwise, the camera might he described as of box form, fixed focus, magazine pattern, containing two or more film slides, which may each hold from i8o to 600 feet of film, and may be changed in daylight for others as required. A great stimulus has been given to the manufacture of lenses of very large aperture for the purposes of the cinematograph, working at from f/3.5 to and with a very short focus, so that pictures are often taken successfully when the atmospheric conditions are far from happy. The shutter is usually of the fan focal plane variety, adjustable to different widths of slit, and some makers claim for their shutters that they will give, when necessary, exposures as short as sec. Most instruments are sighted by means of the view-finder. However, a recently introduced camera is fitted with the reflex system for focussing the image.