Richardson's handbook of projection (1930)

1106 HANDBOOK OF PROJECTION FOR Moral : That seal was placed there for a most excellent reason. Let it severely alone! I have set this forth at length for the reason that some projectionists have complained about the assemblage being sealed in place. They have the idea that they should be permitted to remove it for cleaning, et cetera. As I have said, the assemblage is dustproof. Neither oil nor anything else can get into it and its replacement in perfect adjustment would be a practical impossibility for the projectionist. If the assemblage seems in need of attention, have the service engineer attend to it. Under no conditions would the projectionist be warranted in breaking the seal. We have now examined the slit assemblage and have seen how it projects upon the sound band a thin line of light in exact horizontal register with the lines upon the band, so that only the individual line passing under the light is illuminated. Perhaps a more understandable way of putting it would be to say that as the sound band passes over the sound aperture, only a space upon it .080 of an inch wide by .001 of an inch thick, or "high," is illuminated, which is less than the actual dimensions of the slit, the reduction being accomplished by the slit projection lens and the .080-inch wide aperture in the aperture plate over which the film passes. We will now go a step further and see what this is all about. We have seen that the sound band is illuminated by a line of light of the dimensions just set forth. We also know that this line of light has a steady, unvarying brilliance when it reaches the film. On the other side of the film it, however, is very different. The line of light has passed through the sound