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The Optical Magic Lantern Journal and Photographic Enlarger.
73
at regular intervals. They tend to pull the web along at each operation by an amount equal to the length of the full-faced portion 62 of the pulley 42. This amount is equal to or slightly in excess of the greatest amount of feed which the variable feed apparatus can give in
same driving belt 15, which of course must
: travel at the same speed throughout.
the interval of time of one revolution of the |
pulley 42. The pulley 41 has its face felted, so that the two pulleys will have only a yielding grip on the film, and can slip over it after it has been pulled taut.
To permit this action a certain amount of slack must exist in the web 33 between the
Fig. XXI.
intermittent feed apparatus and the take-up apparatus, which operates continuously. This is indicated in the lower portion of Figs. XXI., XXII, XXIII. of the drawings. The web then passes through the guide 43 and around the pulleys 22 and 23, in contact with another portion of the driving belt 15. It then passes around the pulley 44 to the reel. It is evident that the variable feed mechanism and the takeup mechanism must operate at exactly the same speed under all conditions. In both of these two mechanisms the film is fed by contact with what are simply different portions of the
It is evident also that the intermittent feed given the web of film must exactly equal the distance between the centres of successive pictures on the film, otherwise the pictures will gradually creep out of registry one way or the other, and instead of throwing the whole of each picture on the screen, portions of two successive pictures will be thrown on the screen and the illusion destroyed. It is impossible to pre
determine this distance with exactness, and it varies in different films, in the same film at
| |
Fig. XXII.
different times, and even in different portions of the same film at any one time, by reason of the uneven expansion and contraction of the film in drying and under different atmospheric conditions. This accuracy of registry is secured by making the intermittent feed of the cut away pulley 42 slightly in excess of the greatest distance between centres of adjacent pictures and placing the variable feed mechanism under exact control of the operator, who has his hand upon the crank shaft 9. If he sees that the pictures are creeping off the screen in a direction which indicates that the web 33 is not travelling