452 PHOTOELECTRICITY, ART AND POLITICS 
However, actually sound-on-disc began soon to be replaced by sound-on- 
film, and to-day is almost completely obsolete. Sound-on-film means that 
there is printed on the film, alongside the picture, a strip of transmission 
varying with the vibration of the recorded sound. Light from the pro- 
jector—usually a subsidiary projector—passes through this strip and falls 
on a photoelectric cell, the varying current for which is fed through ampli- 
fiers to a loudspeaker The reasons why sound-on-films has replaced 
sound-on-disc are two. First, that recording sound-on-film is a photo- 
graphic process ; motion picture engineers knew all about photography, 
but they did not know about the highly specialised art of making gramophone 
records. But the second is even more important. When sound-on-disc 
is used, the record is on two separate objects, which is inconvenient for 
many purposes ; when sound-on-film is used, it is all on one, and there is 
no fear of the two parts of the record becoming dissociated in any way. 
Here we are going to leave for a moment actual history for imaginary, 
and point out to you what great and curiously indirect effects might have 
flowed from small causes. It is the advantage of sound-on-film that the 
record is a single object. But that is not an unmixed blessing in an inter- 
national industry. The same picture will do in all countries, but not the 
same sound; language differences have to be taken into account. Now 
suppose that the Hollywood magnates had been far-sighted visionaries or 
even tyrants, consistently intent only on their own advantage—perhaps 
that is more likely and equally effective ; and suppose they had said ‘ we 
are going to have none of this silly Tower of Babel business. If people 
want to see our films, they’ve got to learn our language. We’ll make it 
easy for them and use some simplified form of English or perhaps even 
Esperanto.’ Don’t you think they might probably have got away with it 
and imposed upon all the earth an international language ? Remember how 
the idioms of Hollywood have permeated our speech and how many of us, 
in a short five years, have become almost bilingual, speaking American but 
still reading English. Perhaps you are not quite sure whether the conse- 
quences would have been good or bad; but there is no doubt that they 
would have been extremely important and have had enormous political and 
social repercussions. The whole history of mankind might have been 
unforeseeably influenced by some long-neglected laboratory toy exploited 
at last for some entirely irrational purpose. 
Now let us return to fact. There is another aspect of this unexpected 
development of photoelectricity. Photoelectric cells produced the sound 
film ; but the sound film also produced photoelectric cells by creating a 
demand for them. Hitherto cells had been made in ones or twos by labora- 
tory workers ; now they were demanded in thousands ; it became worth 
the while of large industrial concerns to manufacture them and to apply to 
the problem their vast experience in similar fields. The emission cells 
received particular attention, for these had many advantages over con- 
ductive cells for talking pictures; and the problems they presented were 
similar to those of incandescent lamps and thermionic valves which had 
been for years the main concern of industrial research organisations. Ina 
very short time the first considerable advance had been made since the days 
of Elster and Geitel, thirty-five years before ; the sensitivity of emission cells 
was increased greatly, and sensitivity—the current due to a given amount 
of light—was still important in spite of our new powers of amplification. 
But such improvements alone would not have extended greatly the field 
of photoelectric applications ; the obstacle to extension was not lack of 
technical power, but simply ignorance on the part of those who might use 
it. Once photoelectric cells became the concern of the large electrical 
