The brightness of a surface is measured in lamberts or millilamberts. A lambert is "the brightness of a perfectly diffusing surface radiating or reflecting one lumen per square cm." A millilambert is 1/1000 lambert. For further definitions the reader is referred to the reports of the committee on nomenclature of the Illuminating Engineering Society. Dubois (1886) states that one of the prothoracic organs of _Pyrophorus noctilucus_ has a light intensity of 1/150 Ph[oe]nix candle of eight to the pound (probably about equivalent to 1/150 candle) and that 37 or 38 beetles (each using all three light organs) would produce light equivalent to one Ph[oe]nix candle. Langley (1890) found that to the eye the prothoracic organ of _Pyrophorus noctilucus_ gave one-eighth as much light as an equal area of a candle and the actual candle-power of the insect was 1/1600 candle. It may be remarked in passing how widely divergent these observations are. For the flash of the firefly (_Photinus pyralis_) Coblentz (1912) found variation from 1/50 to 1/400 candle, the predominating values being around 1/400 candle. A continuous steady glow is sometimes obtained from this insect and it proved to be of the order of 1/50,000 candle. Steady sources of light can be more easily measured and we have two records of the light intensity from luminous organisms with continuous light. One of these is a fish, _Photoblepharon palpebratus_, with a large luminous organ under the eye, of flattened oval shape, 11 × 5 mm., which glows continuously without change of intensity. The organ can be darkened by a screen similar to an eyelid which pulls up over it. Steche (1909) reports the intensity to be .0024 M.K.[3] [3] The metre-kerze is a unit of illumination, not of intensity, and is incorrectly used by Steche. Luminous bacteria probably glow with less intensity than any other organism. The light from a single organism cannot be seen but that from a colony is visible to the dark-adapted eye. Even so we must remember that the eye is an exceedingly delicate instrument which can detect very small energy changes. The "minimum radiation visually perceptible" has been calculated by Reeves (1917) to be in the neighborhood of 18 × 10^{-10} ergs per second and the light from a small colony of luminous bacteria represents little more radiation than this.