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240 PROGRESS REPORT [j. s. M. P. £.
A compressed air control is used in the Spoor-Bergren projector to hold the 56 mm. film flat.155 Ritterath156 is inclined to believe the secret of stereoscopic motion pictures lies in the use of a composite screen rather than special cameras or projectors. A relatively complicated method of stereo-motion pictures having severe practical limitations has been described in a paper by Ives157 before the Optical Society of America. A few patents related to stereoscopic motion pictures have been granted.158
Non-intermittent Projection. — Efforts continue to perfect nonintermittent projectors, most of the published results coming from Germany. The most successful of the commercial models, the Mechau projector has been further improved in Model 4, a description of which appeared in an issue of Kinotechnik early this year.159 Burmester and Mechau160 have prepared a very complete treatise on the mechanical and optical principles underlying this projector. Several German theaters are reported to be using them and an earlier model was installed for a short time at the Capitol Theater in New York a few years ago. Thun161 has published a paper on projection with optical compensation and Hatschek162 has described a non-intermittent projector of comparatively simple construction which utilizes a spiral concave mirror with an inner hollow face which rotates on a vertical axis once per picture. The first paper of a series written on various non-intermittent projector systems has appeared recently.163 It describes the projector invented by Nilson in which one pair of oscillating mirrors are used to project the image in place of the usual large number of mirrors and prisms. Several patents164 have been taken out on optically compensated projectors which describe among others the use of a rotating disk (set at 45 degrees to the film plane) in the periphery of which mirrors are placed; the employment of a rotating polygon of refractors having plane parallel surfaces; and the use of mirrors carried by two rotors and arranged prismatically thereon, the number of mirrors being different on the two rotors.
Portable Projectors. — A sound-on-film portable projector has been marketed by Western Electric. A mechanical governor controls the continuous movement of the film past the photo-electric cell and sound lamp.165 Various devices166 have been patented for handling endless film bands in projectors, coin-in-slot operated projectors, and translucent screens. One patent describes a music roll on which are printed a series of pictures which show as motion pictures when the roll is rewound rapidly.167