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AN HISTORICAL STUDY 453 
firms, they were brought to the attention of all electricians. Slowly they 
began to be regarded, not as curiosities arousing the enthusiasm of a few 
specialists, but as normal tools at the disposal of the normal engineer for 
any purpose to which they might be adapted. ‘The end of that stage has 
not quite been reached ; there is still a group of enthusiasts whose one idea 
is to find some use for photoelectricity even when alternative methods are 
obviously preferable ; and another group of the unconverted who regard 
them as unproved novelties. But the intermediate group is steadily gaining 
ground ; at last originality and sound judgment are at work together. And 
that—mind you—is largely due to talking films ! 
Let us then inquire soberly and in the light of our present knowledge, 
what is the proper field of photoelectricity. ‘The advertiser loves to descant 
on the marvels of the electric eye ; and that phrase, which can hardly have 
escaped your attention, suggests that the proper function of the photo- 
electric cell is to replace vision. Now that is quite wrong. The eye may 
be physiologically the equivalent of a vast assembly of photoelectric cells 
connected to an inconceivably complicated automatic telephone exchange 
located in the brain. But any practicable combination of cells and accessories 
share with the eye only one common power, that of distinguishing light from 
darkness. The cell lacks altogether the power of the eye to appreciate 
directly form and colour, and on that appreciation most of the uses of vision 
depend. On the other hand the cell possesses powers that the eye lacks ; 
it can detect much smaller variations in illumination and much more rapid 
variations. 
The first of these powers makes photoelectric cells valuable measuring 
instruments. 'Uhat has been realised from the start. Wernher von Siemens 
made a selenium photometer in 1875 ; Elster and Geitel studied measure- 
ment with great care and insight. (Measurement is amuch more complicated 
matter than most people suppose.) But, except in astronomical observa- 
tories, where the great sensitivity of the cell to small amounts of light was 
valued, photoelectric photometry was not practised seriously before the 
war. Since then its use has spread at an increasing rate. It is often more 
convenient than visual observation ; its convenience has been enhanced 
greatly by the appearance of the rectifier photoelectric cell within the last 
five years. For instance, photographic exposure meters using such cells 
are now on sale for general use. But the quality on which we want rather 
to insist is its accuracy. The importance of accurate measurement in 
industry is not generally understood. ‘Thus, since the usual purpose of 
light is to enable us to see, it is not immediately obvious why any measuring 
instrument more accurate than the eye is necessary or desirable. The 
answer is that variations in quality too small to affect appreciably the 
finished product provide a most valuable clue to defects in manufacture 
which, if they are unchecked, will lead to waste. It does not matter much 
to you whether one lamp that you buy gives one per cent. more or less light 
than another; but it is by keeping track of such small differences that 
manufacturers have steadily improved the quality and diminished the cost 
of lamps by eliminating waste. Here is a function of science that in uni- 
formly beneficial. Even in our mad world, where we try to rectify economic 
disasters by destroying valuable products such as coffee and rubber, waste— 
the expenditure of human energy in achieving undesired results—is surely 
an unmixed evil. The elimination of waste is one of the least spectacular 
achievements of science, and one of which the general public seldom hear ; 
but it is one of the most useful. Here photoelectric cells have much to 
their credit. 
Another potential advantage of photoelectric over visual measurement 
