446 PHOTOELECTRICITY, ART AND POLITICS 
reason. If only the stream of invention and discovery could be slowed 
down, so that the community had time to accustom itself to each new power 
before it was presented with the next, it would be far less likely to abuse 
them. Hence we get pleas for a scientific truce, during which no more 
advances in knowledge should be made. The objection to such a plan is, 
of course, that it is impracticable ; its execution demands a greater, not a 
less, unity and consistency of purpose than some less drastic method of 
control. Prohibition is of all regulations the most difficult to enforce. 
Even if it is true that the suddenness of science is the main source of 
our troubles, it does not follow that a speed limit is the proper remedy. 
A better plan may be to exercise a greater foresight. 
We do not pretend for a moment that it is easy to foresee the direction 
that discovery and invention are likely to take even in the immediate future. 
One of the chief duties of an industrial research laboratory is to assist in 
such forecasts, and we are very conscious how easy it is to make mistakes. 
But, on the other hand, it is not quite so difficult as the public are often led 
to believe. Novelty has its own attractiveness; and people with new 
devices to sell always use the romantic appeal of the great invention springing 
suddenly from the brain of a single genius. But most inventions hailed 
with a blare of trumpets as the latest epoch-making marvel are not matters 
for sentiment. Either they are comparatively trivial modifications of old 
devices, which any well-informed engineer has long known to be inevitable ; 
or they are incomplete suggestions, thrust into prominence long before 
they are due, vanishing from the public memory as quickly and as suddenly 
as they appeared. 
All inventions that influence greatly the course of history have themselves 
a history behind them. And the histories of many inventions have much 
in common. If we want to guard ourselves against surprise in the future, 
we should study the past and apply its lessons. If is for this reason that 
we have been asked to invite your attention to-day to the history of one 
particular invention or discovery. We are not sure that its lessons are 
very obvious ; perhaps we are not in a position to see tham ; the engineer 
himself may not be able to see the wood for the trees and must leave it to 
the trained historian to draw conclusions. We shall try simply to tell the 
story, and shall make only the simplest and most obvious reflections. 
There are several reasons for choosing the photoelectric cell as an 
example. The first is that the history is exceptionally long and complicated. 
That statement may surprise those of you who have just heard of photo- 
electric cells and regard them as one of the marvellous products of the last 
few years. Nevertheless it is true. We shall have to start our history 
sixty years ago, and shall find that the development of cells themselves, as 
distinct from their accessories, was almost complete forty years ago. No 
better example could be offered of how the general public are misled by 
those who are so busy making history that they have no time to read it. 
The second reason is that the results of the applications of photoelectricity 
hitherto have been comparatively trivial. "They have not aided the struggle 
against disease and poverty ; and on the other hand they have not provided 
weapons for war or displaced large quantities of labour. In discussing 
them our passions will not be aroused, and we can consider with detachment 
their potential, and perhaps their future, effects, which might have been or. 
may be considerable. 
And now what are photoelectric cells ? As their name implies they are 
devices for turning light into electricity ; or more accurately, devices in 
which the incidence of light produces or changes an electric current flowing 
in some circuit. We are not going to discuss their action in detail ; for 
