Richardson's handbook of projection (1930)

1104 HANDBOOK OF PROJECTION FOR the side of the assembly cell is a small observation port, or window, through which the projectionist may observe the slit spot, and so adjust it that it centers upon and properly covers the slit. THE SLIT. — What is known as the "slit" is just what its name implies. It is formed by two knife edges placed exactly parallel with each other, located one and one-half (lj^) thousandths of an inch apart. As you will see by referring to Fig. 385, it is located approximately midway between the condenser and the objective lens of the slit assemblage. It, the slit, is three-sixteenths of an inch in length. Now get the following clearly fixed in your mind: As has been related, the similar slit through which the sound band is illuminated at the camera was precisely at right angles with an imaginary line drawn lengthwise through the center of the sound track. That meant that as the sound track passed through this thin line of light, the varying densities of impression left upon the sound track by the light were in a horizontal plane and appeared after development in the form of lines exactly at right angles to the above mentioned imaginary line. I think it will be obvious to you that what was impressed upon the sound track at the camera must be picked up in precisely the same wav at the projector sound aoerture. All right ! In order to pick up the photographic impressions exactly as they were impressed, it is obviously necessary that the pick-up line of light register precisely with the line of light which illuminated the sound track. Not only is that true, but also the speed at which the