276 



NATURE 



[October 27, 192 1 



Speaking Films. 



By Prof. A. O. Rankine. 



THE publicity recently given [Times, Sep- 

 tember 24 and 28J to reports of the suc- 

 cessful synchronisation of speech and action in 

 kinematography makes the present an appropriate 

 time for describing the production and use of 

 photographic films bearing sound-records which 

 are reproducible. For the novelty of the recent 

 inventions does not lie in the speaking films them- 

 selves, but in their combination with picture films 

 so as to constitute the so-called " talking pic- 

 tures." It was about the year 1900 that 

 Ernst Walter Riihmer made the first speaking 

 film. The process is described in his book on 

 "Wireless Telephony," translated into English by 

 J. Erskine Murray in 1908. Riihmer's invention 

 was the natural outcome of his work on photo- 

 telephony, the principles of which are treated in 

 the same work. In photo-telephony there are im- 

 posed upon a projected beam of light fluctuations 

 of intensity which correspond to the sound-vibra- 

 tions associated with speech ; for purposes of re- 

 production the light is allowed to fall upon a 

 selenium ^ cell, which, by its well-known photo- 

 electric property, controls the current in a tele- 

 phonic circuit. It was a simple modification to 

 carry out the process in two distinct stages, viz., 

 (i) to photograph the fluctuations of the light 

 upon a moving film, and (2) to actuate the 

 selenium cell at leisure by interposing between it 

 and a source of light, ihe developed film moving 

 at the same speed as before. For this device 

 Riihmer chose the descriptive but ugly name 

 "photographophone," and printed reproductions 

 of some of the films obtained by him appear in 

 that chapter of his book which bears this name. 



Riihmer's method of obtaining the fluctuations 

 of light corresponding to speech was to super- 

 impose upon the current in an electric arc varia- 

 tions due to a microphone actuated by the voice. 

 This method has been used by several later 

 investigators, including H. Thirring (Phys. Zeit., 

 p. 67, 1920), who has also devised a particularly 

 sensitive form of selenium cell. The chief diffi- 

 culty In connection therewith appears to be that 

 of keeping the arc in a sufficiently sensitive con- 

 dition. In an entirely different method, due to 

 the author, described in Nature for February 5, 

 1920 (vol. 104, p. 604), under the title 

 "Telephoning by Light,^'^ this diflficulty does 

 not present itself, and a fluctuating beam 

 of light with the necessary characteristics is 

 obtained with ease. Its application to the pro- 



1 Selenium is not the only substance suitable for this purpose. Other 

 rhoto-electric cells have been constructed during recent years, notably the 

 " thalofide cell" of T. W. Case and the " antimonite cell " due to W. S. 

 Grip-nb-rg. The sensitive substances here involved are sulphides of thal- 

 lium and antimony respectively. The relative merits of such cells are open 

 to question, and require car'ful experimental investigation ; but they do not 

 affect ar all the prinriples of sound-reproduction from photographic films as 

 described in this article. 



2 The article here referred to should be read in conjunction with the 

 present one. 



NO. 2713, VOL. 108] 



duction of speaking films has already been indi- 

 cated (Proc. Phys. Soc, vol 32, p. 78, 1920J, but 

 many new records have been obtained since the 

 date of that publication. 



The mode of recording adopted, which differs 

 in no essential respect from Riihmer's, is to allow 

 the fluctuating beam of light emerging from the 

 photophone transmitter to fall upon a condensing 

 lens, Lj (Fig. ij, so that an image of the original 

 source of light is formed at S. A narrow, hori- 

 zontal slit thus illuminated at S serves as a 

 secondary source, and the lens L^ brings an image 

 of it to a focus upon a continuously moving photo- 

 graphic film suitably enclosed. A shghtly different 

 arrangement, which is superior optically, is to 

 place a larger slit close to, and extending over 

 the full aperture of, the lens Lj, and to obtain 

 on the film a suitably diminished image of the 

 slit by giving the lens Lo the appropriate focal 

 length. By providing in addition that the focal 

 length of Lj is equal to the distance LjLq, the 

 image of the original source, usually quite small, 

 coincides with L2, of which the central region only 

 is thus used, and aberration is much reduced. It 

 will be seen that the moving film is exposed to 



Fig. X. 



a narrow bar of light, perpendicular to the direc- 

 tion of motion, of which the intensity is varying 

 in accordance with those sounds which have 

 actuated the photophone beam. The result is that 

 the film, when developed, shows a band varying in 

 opacity as the length is traversed, and looking 

 very much like a discontinuous spectrum. Two 

 examples (reduced by one-fourth) are shown in 

 Fig. 2 — the records of the words "beet " and 

 "this" respectively. They have been chosen 

 because they are short, staccato words capable j 

 of being reproduced in the space available. The ^ 

 beginning of each word is at the top, and the 

 speed of the film was about 13 metres per second. 

 The amount of detail shown in these records 

 depends, of course, upon the relation between the 

 width of the slit image on the film and the film | 

 velocity. For those shown the slit image was 

 about 02 mm. wide, so that frequencies of several 

 thousand per second, if present, should be visually 

 detectable. 



The procedure for reproducing the sounds from 

 the films is very simple. All that is needed is to 



