S. P. Langley—The Actinic Balance. 189 
degree of accuracy, and although something was gained it was 
too small 
the autumn of 1880, when he found himself in possession of 
an instrument, not only greatly more sensitive than any ther- 
mopile, but also far more prompt, and as he believes more 
accurate, 
hen (to use a common illustration), the finger is applied 
to the trigger of a gun, the little force liberates an indefinitely 
greater one, which has no certain relation to the energy of the 
original impulse. But what we here need is a rigorous propor- 
tonality between the feeble but momentarily varying energy 
f the original ray, and the amount of power it releases from a 
attery or other source of energy. It is only on these condi- 
hons that the indications of our instrument will be accordant 
and that it will be truly a meter. If we are in search only for 
xtreme sensitiveness, and are satisfied to have a delicate ther- 
