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.
Lode (1904, 1908), by a modified grease spot photometer method, ascertained that the light of his brightest bacterial colony of Vibrio rumple had an intensity of 7.85 × 10-10 H.K. per sq. mm. or 0.785 H.K. per 1000 sq. metres (=0.562 German-normal candles per 1000 sq. metres). In round numbers this is about one German-normal candle per 2000 sq. metres, or two to three times this area for the light from an ordinary stearin candle. Lode calculated that the dome of St. Peter's at Rome, if covered with bacteria, would give little more light than a common stearin candle. An ordinary room of 50 sq. metres wall and ceiling area would give out only 0.039 German-normal candle. It does not seem likely that luminous bacteria will ever come into vogue for illuminating purposes. Friedberger and Doepner (1907) by a photographic method, not entirely free from error, found that one square millimetre of lighting surface of a bouillon culture
of photobacteria gave 6.8 × 10-9 German-normal candles, about ten times Lode's value. Even at this rate commercial lighting by luminous bacteria does not appear a promising field for investors.
To sum up, we may say that light from animal sources is in no way different from light of ordinary sources, except in intensity and spectral extent. It is all visible light, containing no infra-red or ultra-violet radiation or rays which are capable of penetrating opaque objects. It is not polarized as produced, but may be polarized by passing through a Nichol prism. Like ordinary light, animal light will also cause fluorescence and phosphorescence of substances, affect a photographic plate, cause marked heliotropism of plant seedlings (Nadson, 1903) and stimulate the formation of chlorophyll (Issatschenko, 1903, 1907). Because of the weakness of bacterial light, etiolated seedlings do not become green to the eye (Molisch, 1912 book), but a small amount of chlorophyll is formed which can be recognized by the spectroscope because of its absorption bands.
CHAPTER IV
STRUCTURE OF LUMINOUS ORGANS
The production of light is the converse of the detection of light. In the first case chemical energy is converted into radiant energy; in the second case radiant energy is converted into chemical energy. The lantern of the firefly is an organ of chemi-photic change; the eye is an organ of photo-chemical change. While it is theoretically probable that all reactions which proceed in one direction under the influence of light, will proceed in the opposite direction with the evolution of light, the formation of luciferin from oxyluciferin (described in Chapter VI) is the only one definitely known. Perhaps we may place in this category also the instances of photoluminescence, but the chemical reaction involved cannot be pointed out.