International projectionist (Jan 1943-Dec 1944)

good part of the sound would be lost. If in front of this board you placed a second baffle board, with a similar slit arranged vertically, then left-to-right and up-and-down vibrations would be intercepted, only back-and-forward vibration could get through. If there were no other path for the sound around the edges of the boards none would get through. Polaroid consists of crystals (embedded in a transparent plastic material) which act like these slits in the baffle boards just imagined, in that they pass light vibrations when the vibratory motion is in certain directions only. Hence if light is passed through polaroid, vibratory motion up-and-down (let us say) is blocked. If then another layer of polaroid is placed in the path of the light in such a way that left-to-right vibratory motion also cannot get through, practically all the light is stopped. This phenomenon is one of many that are taken to prove that light consists of, or is accompanied by, vibratory motion, though what particles undergo that motion is not known. Whatever they are, they are apparently the same as the undiscovered particles that comprise electro-magnetic fields, for light obeys the same mathematical laws as high frequency electro-magnetic phenomena. There appears to be a vast spectrum of vibrations, the lowest in frequency being the electro-magnetic field that surrounds a conductor carrying low-frequency a.c. As the frequency is increased the field takes on the properties of a radio field — beginning as low as 30,000 cycles or thereabout it can be detected over vast distances with suitable "receiving" apparatus. Beginning just beyond the highest radio frequencies vibrations of apparently identical kind manifest themselves as heat. The highest frequencies of heat, in turn, merge imperceptibly into the lowest frequencies of light — the infra red. Light, again, is said to have many wave-lengths (to most of which the eye does not respond). There is a whole band of (invisible) infra-red frequencies. The lowest frequency of light that can be seen is red; successively higher frequencies range through yellow and blue to violet — where the eye again gives up. Beyond the visible violet, still higher in frequency, there is the family of ultraviolet frequencies. Beyond these again, X-rays and cosmic rays. Sound waves do not belong to this group of vibrations. These electro-magnetic waves vibrate in an unknown medium. The medium of ordinary sound vibrations is familiar air; the particles that vibrate are the molecules of which air is composed. Sound will also travel through water, a metal rod, etc., in which case the molecules of water or of the metal undergo vibration at sound frequencies. When a loudspeaker operates vibrations of identical frequency and amplitude occur in at least two different mediums simultaneously; in the electromagnetic field surrounding the speaker coil and in the air surrounding the speaker diaphragm. ( To be continued) MOTIOGRAPH EXPANDS To better facilitate the handling of extensive war orders Motiograph, Inc., manufacturers of projectors and sound systems, has not only gone on a 24-hour work basis but has also acquired an additional plant. Completion of a navy contract for sound and projection equipment will be followed by the production of aircraft and ordnance components, to which the company will devote 100% of its facilities. PROJECTOR MAKERS HELP SLASH TRAINING TIME High praise for the war contributions of Chicago camera and projector manufacturers is contained in the recent issue of Domestic Commerce Weekly, publication of the U. S. Department of Commerce. "Certain activities of the Army and Navy,. as well as civilian defense, health and nutrition programs, agricultural production and other essentials, not excluding morale," writes Walter A. Foy, consultant of the Department's Chicago area, "depend upon the film as the primary educational me YOU KEEP YOUR PRESENT EQUIPMENT I N SERVICE . . . until that time when you can purchase PROJECTION LAMPS NATIONAL THEATRE SUPPLY COMPANY "THERE'S A BRANCH NEAR YOU" FEBRUARY 1943 IT