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Feb., 1930] PROGRESS REPORT 231
duction and believes that acting will show improvement under the supervision of sound recording.
The Gaumont method of sound recording using a sound record which is transparent to visible light but absorbs the ultra-violet, as mentioned in the last report, has been described more completely and protected by an additional patent.66
Schinzel67 has reviewed various methods of producing sound records on films which transmit visible radiation but absorb ultra-violet.
Another variable area method of sound recording has been described by Miller.68 It is known as the Vita vox which utilizes a light valve, an image of which is focussed directly on the film. The methods of recording used by Tobis (Ton Bild Syndicate) have been dealt with by Bohm.69 The position of the sound record which was originally outside the perforation area has been changed to make it conform with common practice. The Selenophon is still another variable area method of sound recording which appears to have promise. Hatschek70 states that a torsion galvanometer is used, the image of an illuminated slit on the vibrating band being magnified 100 times for recording on the film. For reproduction a "condenser-less" type of selenium cell is used. The lag of the cell is stated to be compensated for largely by the suitable use of amplifiers.
The technic of electromagnetic recording has been studied by a German pioneer in this field, Curt Stille,71 who claims that besides having most of the advantages of optical recording the method avoids all difficulties inherent to film such as resolving power, development troubles, and printing errors. Play-back can be made immediately and the record is not subject to scratches. Records 15 years old were found to be as good as when first made.
The British Acoustic film system utilizes separate films for the picture and the sound record which latter is of the saw-tooth type.72 A selenium cell is employed in the reproducer, the lag being offset by using a filter circuit which attenuates the lower frequencies in correct proportion.
There has been further progress in voice doubling so that it can on occasion be done to a very high grade of perfection, but a public reaction against it has developed in those cases where it has become known. The practice is, therefore, being very rapidly discontinued.
At the same time, a somewhat related method is very promising which involves recording the sound and scene with the same actors, but at different times. This method is not open to the objection