The Nature of Animal Light

by E. Newton Harvey

A similar conclusion is reached if we attack the problem in another way. X-rays and radium rays (Becquerel rays) cause fluorescence of ZnS, barium platinocyanide,

willemite (Zn₂SiO₄), and calcium tungstate. Coblentz (1912) showed that the firefly will cause no fluorescence of a barium platinocyanide screen and I have been unable to detect fluorescence of zinc sulphide, barium platinocyanide, zinc silicate (willemite) or calcium tungstate shielded from Cypridina light by black paper, although the light of this organism is quite bright enough to cause phosphorescence of zinc sulphide without the black paper. The samples of the above four substances all showed fluorescence in presence of radium rays, but only the ZnS phosphoresces after exposure to light rays, although the willemite was phosphorescent after exposure to the ultra-violet.

While photometry at low intensities is a difficult procedure at best, if the light varies in intensity or is a flash, accurate measurements become well-nigh impossible. The figures given for intensity of animal luminescence must, therefore, be accepted with a realization of the difficulties of measurement. By candle is meant the international candle, unless otherwise specified, equal to 1.11 Hefner candles (H. K.) 0.1 pentane lamp and 0.104 carcel units. It is a measure of intensity.

Amount of light, or light flux, measured in lumens, is that emitted in a unit solid angle (area/r²) by a point source of one candle-power. One candle-power emits 4π lumens. The latest figure for the mechanical equivalent of light at λ = .566 is .0015 watt (Hyde, Forsyth and Cady, 1919), i.e., 1 lumen = .0015 watt. One watt is 10⁷ ergs (one joule) per second.

The illumination (of a surface) is that given by one candle at one metre, the candle metre (C.M.) or lux. The

surface then receives one lumen per square metre. A metre kerze (M.K.) is the illumination given by one Hefner candle at one metre distance.

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 Phœnix 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 Phœnix 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.