358 
DR. MEYER AYILDERMAN ON CHEMICAL DYNAMICS 
the intensity of the light of a given source of light must he directly proportional to 
the corrected deflection of the galvanometer (or to the tangent of the angle), and 
can be measured by it. 
It should be remarked that this objective method of measuring the intensity 
of light by means of a thermopile (or bolometer), and of checking it by means of 
a Clark cell and known standard resistances, measures the total heat energy 
produced by the given source of light in the thermopile, and thus differs from the 
other methods of measuring the intensity of light, the ordinary photometers, 
based upon the physiological effect of the light upon the eye. No doubt there is a 
distinct difference between the two ; an acetylene light which we physiologically 
perceive, say of 16 candle power, is very much cooler than a 16 candle-power coal- 
gas light. The temperature to which the exposed junctions of the thermopile will be 
raised by the rays of flames of the same candle power, but from different sources of 
light, will therefore be different, the more so as the colour and composition of 
different lights are also different. Properly speaking two lights from two different 
sources (say acetylene and coal gas, or arc light) can neither lie compared physio¬ 
logically on account of their different compositions, nor in the objective way by 
means of a thermopile or bolometer, as given above, and no light can be compared 
except with a light of the same nature. For comparison of two lights of the same 
kind there can be no doubt that the objective method by means of a thermopile or 
bolometer is by far more accurate and reliable than measurement in the physiological 
way. As, however, for many practical purposes, the intensity of different lights has 
almost always to be expressed in candle units, different sources have to be compared 
and measured in this physiological way as far as it is practicable. In this photo- 
meti ic work, at any rate, all the standard units for comparison, the small as well as 
the large ones, ought to be correct multiples of one another, i.e., ought to be 
all from the same source of light and of the same composition. Acetylene, as far as 
we know, is the only source of light which gives reliable standards from very small 
units, such as O'l or 1 candle power (when only a part of the line is used) up to very 
great units, such as 500-1000 candle power (when many lines on a space of about 
2 inches in diameter should be used), and, except the burners, the same arrangements 
can be easily manipulated for fixing and adjusting all the standards in an absolutely 
correct and objective manner, as described here. 
The author now passes to the description of the thermopile and of the other parts 
of the arrangements, indicated by the above diagram, which were used for measuring 
the intensity of the acetylene light. 
The Thermopile (or Bolometer). See fig. 5. 
A detailed description of the thermopile used is given by Rubens in the 
‘ Zeitschrift fur Instrumentenkunde,’ 1898, pp. 65-69, but a few data with regard 
to it must be given here. 
