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ANIMATED PHOTOGRAPHY 371
Porter's equations lead to the conclusion that the frequency of change of picture should lie between 38 and 46.8 per second. This is not possible in practice, since the strength of film does not allow the necessary speed , and the frequency employed lies between 15 and 25. Helmholtz has shown that there is a change in the critical frequency with change of the proportion of the light to the dark interval, and it appears from Marbe's experiments that beyond a certain frequency reduction of the dark interval is accompanied by a decrease in the critical frequency. Taking the value 25 for the frequency, the exposure interval should be ten times longer than the time of motion of the picture. This, however, gives 1/275 second for dark interval, which is too short, and in practice the ratio is reduced from 10 to 4 or 5.
When coloured light is employed the critical frequency is less than for white light, the order of the colours with respect to frequency being the same as that given by Konig's curve of sensitiveness of the eye for the spectrum colours.
Some machines are made so that the effect can be reversed at will by simply turning the handle in the reverse direction. These cause a great deal of amusement, and enable much more to be got out of any set of films.
These large instruments can take films varying from 500 to 2000 feet in length. As a rule, however, films beyond about 800 feet in length are cumbersome, and are very apt to tire the eye. even if the action is smooth and free from flicker. The next two figures (166, 167) show (a) a combination which can be used for exhibiting pictures of still life and also animated photographs, and (b) a reversing machine. In the latter, the lantern is not shown. In the second picture the film can be seen in position. The film track in these projection instruments is so arranged that nothing touches the film except at the edges, thus increasing the length of time the films can be used.