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Feb., 1930] PROGRESS REPORT 239
nounced a dual system unit selling at $3000.00 for use in theaters seating 500 or less.146
A California exhibitor has installed a few sets of ear phones in his theater for the use of deaf patrons; each set being adjustable for volume by means of a small choke coil.147 Descriptions have been published of several types of amplifiers.148 A small vacuum cleaner device is available for removing dust accumulation in delicate electrical parts of sound apparatus.149
A reproducer employing the condenser principle has been described.150 It consists of a slotted aluminum grid which acts as one plate and a thin layer of gold or aluminum leaf glued to a rubber diaphragm serves as the other condenser plate. Several of these grids are attached to the rear of the projection screen. A hydraulic lift is now used for the stage mounting of loud speaker horns.151
A segmented cardboard disk has been announced which can be used to synchronize the picture with a disk record.152 Fader control from the auditorium is possible with an installation used by Metro-Goldwyn-Mayer for road shows in the larger neighborhood theaters.
A committee of technicians has undertaken an investigation to draw up a set of standards for camera and projector apertures according to a report from the Academy.163 A preliminary survey indicates that the majority of theaters showing sound-on-film pictures are using a screen picture that is nearly square. Sliding masks are sometimes used alone or in conjunction with a horizontally movable lens mount. Sometimes a lens of lower focal length is used and an undersized aperture plate thus restoring the 3 by 4 proportion but at a loss of some of the picture. A recent report 153a states that all the large producers on the Pacific coast have agreed to adopt at once the recommendations of the joint committee of this Society and the Academy of Motion Picture Arts and Sciences providing for the use of a standard aperture of one size in all cameras.
Numerous patents have been granted on improvements in sound projection apparatus and accessories.154
Stereoscopic Motion Pictures. — Since the public showing of the Teleview in New York City in 1922, there have been no further developments of commercial interest in stereoscopic pictures until the past summer when wide pictures were introduced. Both Fox Grandeur and the Spoor-Bergren systems are claimed to give the illusion of depth but those who have seen them state it is only a fair illusion. Special lens systems are used in the recording cameras.