Candle-power is the measure of the luminosity of a source of light by comparison with a definite official unit known as the standard candle. This unit of light is supposed to be produced by a candle one-sixth of a pound in weight, and made to burn 120 grains of spermaceti wax per hour. The length of such a candle is from 8-1/4 to 9 inches, varying slightly with different makers; its diameter is from .8 to .9 of an inch. Unfortunately spermaceti is not a definite chemical compound, and its composition varies. The Acts of Parliament relating to the subject do not define this, nor do they specify the number and size of the threads in the wick. On these and other accounts the standard candle is not a fixed unit, a difference of as much as 25 per cent. being observable in the light of two specimens. The French official standard is the carcel, which is a hollow-wick lamp burning purified colza oil, and giving a light of about 9.5 candles.