Part 4 (2/2)
[Ill.u.s.tration: FIG. 10.--Transmissivity of the integument of fireflies to infra-red radiation (_after Coblentz._)]
Although photographs of the spectrum of firefly (_Photinus_) light show that it extends only to the beginning of the blue, Forsyth (1910) reports ultra-violet radiation in luminous bacteria. He exposed a plate for 48 hours to the spectrum of bacterial light dispersed by a quartz prism and got a continuous band from ? = 0.50 (the lower limit of sensitivity of the plate) to ? = 0.35. However, McDermott (1911 _d_) was unable to observe fluorescence of p-amino-ortho-sulpho-benzoic acid, which responds to the ultra-violet light. Molisch (1904, book) photographed bacterial and fungus light through gla.s.s and through a piece of quartz and found no difference in density on the plate. As the exposure was brief, to avoid saturation, and as the ultra-violet, which pa.s.ses quartz but not gla.s.s, has a much greater action on the plate than visible light, we must conclude that ultra-violet is absent. Ives (1910) investigated the spectrum of _Photinus pyralis_, using a quartz spectroscope, and found no evidence of ultra-violet radiation, at least as far as ? = 0.216.
It will thus be seen that the radiation from the firefly has been very carefully studied and that no waves are given off from ? = 1.5 to ? = 0.216 with the exception of the short band (? = 0.67 to ? = 0.51) in the visible, and it is highly probable that no radiation is given off with wave-lengths longer than ? = 1.5. The firefly light remains, then, 100 per cent. efficient, differing from all our artificial sources of light, the best of which does not approach this value. As Langley and Very express it in the t.i.tle to their paper, it is ”the cheapest form of light,” not cheapest in the sense of that we can reproduce it commercially at less cost than other lights, but cheaper in the sense that it is the most economical in the energy radiated. This energy is all light and no heat. ”Cold light” has actually been developed by the firefly and concerning which ”we know of nothing to prevent our successfully imitating.”
[Ill.u.s.tration: FIG. 11.--Spectral energy curves of various fireflies and the carbon glow lamp (_after Coblentz_).]
I have already pointed out that we may also consider the efficiency of a light in relation to the sensibility of our own eye. That is, we take into account not only the energy distribution in the spectrum of the light but also the fact that different wave-lengths of an equal energy spectrum affect our eye very differently. As the normal light-adapted eye is most sensitive to yellow green of ? = 0.565, monochromatic light of this wave-length will appear much brighter than monochromatic light of any other wave-length with the same energy. Monochromatic light of ?
= 0.565 will then be the theoretically most efficient possible, when we consider the energy radiated in relation to the sensitivity of our eye.
This is the usual method of determining the luminous efficiency of artificial lights and is obtained from a knowledge of the radiated energy and the visual sensibility. Reduced luminous efficiency = light (_radiated energy_ _visual sensibility_) or luminosity total radiated energy.
[Ill.u.s.tration: FIG. 12.--Visibility curves of various investigators obtained by different methods (_after Hyde, Forsyth and Cady_).]
[Ill.u.s.tration: FIG. 13.--Luminous efficiency of the 4-watt carbon glow lamp, shaded area total area (_after Ives and Coblentz_).]
[Ill.u.s.tration: FIG. 14.--Luminous efficiency of the firefly, shaded area total area (_after Ives and Coblentz_).]
The spectral energy curve for the firefly has been worked out by Ives and Coblentz (1910), using a photographic method in which the intensities of different wave-lengths of the firefly (_Photinus pyralis_) light is compared with that of a carbon glow lamp by measuring theamount of photochemical change produced on panchromatic photographic plates. Fig.
11 gives the energy curves of various fireflies and the carbon glow lamp in the same spectral region. The visual sensibility curve used by Ives and Coblentz is that of Nutting (1908, 1911), based on Konig's data. It is reproduced in Fig. 6. The latest visibility curve is that of Hyde, Forsyth and Cady (1918), reproduced in Fig. 12. It is based on observations of twenty-nine individuals. As individuals vary considerably in their sensibility to different wave-lengths, the visibility curve represents an average, but it is the only standard we have with which to evaluate the energy we call light. Color-blind individuals would have a visibility curve very different from normal individuals. Composite curves showing the luminous efficiency of the 4-watt carbon glow lamp and the firefly, both in relation to visibility, are given in Figs. 13 and 14, respectively. In these figures the luminous efficiency is the shaded area total area, 0.43 per cent. for the carbon glow lamp and 99.5 per cent. for the firefly, ”these numbers representing the relative amounts of light (measured on a photometer) for equal amounts of radiated energy--a striking ill.u.s.tration of the wastefulness of artificial methods of light production. From the specific consumption of the tungsten lamp (1.6 watts per spherical candle) and the mercury arc (.55 watts per spherical candle) we obtained by comparison with the carbon filament that their luminous efficiencies are 1.3 and 3.8 per cent. The most efficient artificial illuminant therefore has about 4 per cent. of the luminous efficiency of the firefly.” This is calculated to be .02 watts per candle. More recent determinations (Coblentz, 1912), using a new sensibility curve of Nutting's (1911) for a partially light-adapted eye, give the reduced luminous efficiency as 87 per cent. for _Photinus pyralis_, 80 per cent. for _Photinus consanguineus_ and 92 per cent. for _Photuris pennsylvanica_.
[Ill.u.s.tration: FIG. 15.--Spectral energy, luminosity and visibility curves (_after Gibson and McNicholas_) A. Spectral energy curve of Hefner lamp.
B. Spectral energy curve of acetylene flame.
C. Spectral energy curve of tungsten (gas-filled) glow lamp.
D. Spectral energy curve of black body at 5000 absolute (sunlight).
E. Spectral energy curve of blue sky.
H_g_. Spectral energy curve of Heraeus quartz mercury lamp.
L_v_. Visibility curve for human eye.
L_a_. Luminosity of Hefner lamp.
L_e_. Luminosity of blue sky.
The luminous efficiencies of various forms of artificial illuminants have been calculated by Ives (1915) and are given together with that of the firefly in Table 6. Fig. 15 gives spectral energy curves for various illuminants reduced to 100 at ? = .590, luminosity curves for the Hefner lamp and blue sky, and a visibility curve worked out by Coblentz and Emerson (1917) from observations on 130 individuals.
TABLE 6
_Luminous Efficiencies of Various Illuminants_
------------------------+------------------------+--------+----------------
Efficiency Illuminant and
Commercial
Lumens
(visible commercial description
rating
per
radiation
watt
visual
sensibility
total radiation) ------------------------+------------------------+--------+---------------- Carbon incandescent lamp
4 watts per mean
2.6
0.0042 oval anch.o.r.ed (treated)
horiz. c.
filament
Tungsten incandescent
1.25 watts per mean
8.0
.013 lamp, vacuum type
horiz. c.
Mazda, type c
600 C. P. 20 amp.,
19.6
.032
0.5 w. p. c. Series
type C.
Carbon arc (open)
9.6 amp. clear globe
11.8
.019
Open arc, yellow flame,
10 amp. D. C.
44.7
.072 inclined trim
Quartz mercury arc
174-197 volt, 4.2 amp.
42.0
.068
Gla.s.s mercury arc
40-70 volt, 3.5 amp.
23.0
.037
Nernst lamp
4.8
.0077
Acetylene
1 L per hr. consumption
.67
.0011
Petroleum lamp
.26
.0004
Open flame gas burner
Bray 6 high pressure
.22
.00036
Incandescent gas lamp,
.350 lumens per
1.2
.0019 low pressure
B. T. U. per hr.
Incandescent gas lamp,
.578 lumens per
2.0
.0032 high pressure
B. T. U. per hr.
Firefly
629.0
.96 ------------------------+------------------------+--------+----------------
The firefly light by the above method of calculating efficiency is not 100 per cent. efficient because its maximum (? = 0.567) does not correspond with the maximum sensibility of the eye (? = 0.565), but taking into consideration also other effects of color, the firefly light would be a still more inefficient and trying one for artificial illumination, as all objects would appear a nearly uniform green hue.
Indeed the distortion would be even greater than with the mercury arc, whose objectionable green hue is so well known. ”We may say, therefore, that the firefly has carried the striving for efficiency too far to be acceptable to human use; it has produced the most efficient light known, as far as amount of light for expenditure of energy is concerned, but has produced it at the (inevitable) expense of range of color. The most efficient light for human use, taking into account both color and energy-light relations.h.i.+ps, would be a light similar to the firefly light containing no radiation beyond the visible spectrum, but differing from it by being white.” (Ives, 1910.) Although the spectral energy curve for _Cypridina_ light has not been worked out, it will be noted that the _Cypridina_ spectrum is much longer than that of the firefly, more nearly approaching the spectrum of an incandescent solid giving white light. It approaches, but does not attain the ideal.
Although Muraoka (1896) and Singh and Maulik (1911) have described radiations coming from fireflies which would pa.s.s opaque objects and affect a photographic plate, and Dubois reports the same from bacteria, the existence of such radiation has been denied by Suchsland (1898), Schurig (1901) and Molisch (1904 book). The experiments of Molisch on luminous bacteria are of greatest interest, for they are very carefully controlled and show without a doubt that black paper or Zn, Al, or Cu sheet will allow no rays from these organisms to pa.s.s that will affect a photographic plate, even after several days' exposure. The _visible_ light of luminous bacteria will affect the plate after one second exposure. Moreover, Molisch has pointed out the errors of those who claim to have found penetrating radiation in luminous forms. It seems that certain kinds of cardboard, especially yellow varieties, or wood, will give off vapors that affect the photographic plate. The action is especially marked with damp cardboard at a temperature of 25-35 C., and Dubois and Muraoka must have used such cardboard to cover their plates. A piece of old dry section of beech or oak trunk, placed on a photographic plate for 15 hours in a totally dark place, will register a beautiful picture of the annual rings of growth, medullary rays, junction of bark and wood, etc. Russell (1897) had previously found that many bodies, both metals and substances of organic origin (gums, wood, paper, etc.), placed in contact with photographic plates, would affect them, and concluded that vapors and not rays were the active agents. As a dry piece of wood has a very definite smell, there is something given off which can affect our nose and there is no reason why it should not change, by purely chemical action, the photographic plate. This action of wood on the plate is prevented by interposing a sheet of gla.s.s.
Frankland (1898) has described similar vapors coming from colonies of _Bacillus proteus vulgaris_ and _B. coli communis_ which affect a photographic plate laid directly over the colonies in an open petri dish. There is no effect if the gla.s.s cover of the petri dish is between plate and bacteria. There is, then, no specific emission of X-rays or similar penetrating radiation from luminous tissues which will affect the photographic plate through opaque screens.
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 (Zn_{2}SiO_{4}), 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 s.h.i.+elded from _Cypridina_ light by black paper, although the light of this organism is quite bright enough to cause phosph.o.r.escence 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 phosph.o.r.esces after exposure to light rays, although the willemite was phosph.o.r.escent 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/_r_^2) by a point source of one candle-power. One candle-power emits 4p lumens. The latest figure for the mechanical equivalent of light at ? = .566 is .0015 watt (Hyde, Forsyth and Cady, 1919), _i.e._, 1 lumen = .0015 watt. One watt is 10^7 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 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 pa.s.sing 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.
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