International Review of Educational Cinematography (Jan-Dec 1931)

— 988 — protective apparatus which detract the voltaic light from the porcelain insulators in order to prevent their being damaged (Figs. 9 and 10). In the opposite sense the time accelerator is used to illustrate very slow processes. An example of this is to be found not in the chronicles of machine construction, but in those of natural history in a flower-growing film, which attracted much attention some years ago and in which the blossoming and growing of flowers covering a period of several days and weeks may be shown in a few short minutes. These apparatus have also been used to show in the course of a single evening the construction of large buildings, which in themeselves take months and years. The second great field of the film of investigation is the examination of materials, which is often not merely superficial, but penetrates into the Fig. 4. texture. Such investigations concerning tension in the tearing asunder of materail have been made bv putting strips of celluoid or small glass listeis into the machine, exposing them to a penetrating light and filming in this way the object to be tested. The tensions inside the material were marked on the film in the form of dark spots, which increased in size until the moment of the splitting of the material. In order to make practical use of these experiments, the laws of similarity in the figures of expansion must be observed. It is, for instance, possible to estimate from the tensions in celluoid what tensoins will be in the splitting of steel, the same applies in comparisons of resistance between glass and cast iron. Films, illustrating the momentary condition of a technical item, for instance of pulleys or pipes, also have instructive or documentary value. Special instruments have also been manufactured exclusively for these purposes. By means of them it is possible to test automatically whether there