(xanthin or uric acid). One might surmise that the light of the animal
was a crystalloluminescence accompanying the formation of these
crystals. It is easy to show, however, that the light comes not from the
crystal layer but from another layer of cells containing large granules.
It is also dependent on the presence of oxygen while
crystalloluminescence takes place in the absence of oxygen. The crystal
layer possibly serves as a reflector. Its significance will be
discussed in a later chapter.
[Illustration: FIG. 5.--Dubois's figures showing transformation of
photogenic granules to crystals (_after Dubois_).]
The light of luminous organisms is quite generally associated with
granules. In one of the centipedes (_Orya barbarica_), which produces a
luminous secretion, Dubois (1893) has described the transformation of
these granules into crystals and at one time he supposed the light to be
a crystalloluminescence. He later reversed this opinion and, certainly,
examination of his drawings which are reproduced in Fig. 5 does not
convince one of the actuality of crystal formation.
The phenomenon of _lyoluminescence_, described by Wiedemann and Schmidt
(1895) as a light accompanying the solution of colored (from exposure to
cathode rays) crystals of Li, Na, or K chlorides, is probably due to a
triboluminescence from stirring of the crystals during solution.
CHEMILUMINESCENCE.--As the name implies, chemiluminescence is the
production of light during a chemical reaction at low temperatures. This
does not mean that the other types of luminescence are not connected
with chemical reactions--using the word _reaction_ in a broad sense--for
we have reason to believe that in some cases spectra are not
characteristic of the element as such but are rather characteristic of a
particular reaction in which the element takes part (dissociation into
ions, changes from monovalent to bivalent condition, etc.) and that this
is the reason one element may show various spectra under different
conditions (Bancroft, 1913). The chemiluminescences are rather oxidation
reactions involving the absorption of gaseous or dissolved oxygen and
may be very easily distinguished from all the previously mentioned
luminescences by this criterion. They should, perhaps, more properly be
called _oxyluminescences_.
The glow of phosphorus is the best known case, recognized since
phosphorus was first prepared by Brandt in 1669. It is interesting to
note that when first prepared phosphorus was regarded as a peculiarly
persistent type of phosphor, _i.e._, a material akin to the impure
alkaline earth sulphides.
Fresh cut surfaces of Na and K metal will glow in the dark for some
time, especially if warmed to 60 deg.-70 deg. (Linnemann, 1858). A film of oxide
is formed over the surface, showing definitely that oxidation has
occurred. Ozone oxidizes organic matter with an accompanying glow
(Fahrig, 1890; Otto, 1896). The light from ozone acting on pyrogallol
solution is especially bright under certain conditions.
Radziszewski (1877, 1880) gives a long list of substances, chiefly
essential oils, which luminesce if slowly oxidized in alcoholic
solutions of alkalis. Formaldehyde, dioxymethylen, paraldehyde,
metaldehyde, acrolein, disacryl, aldehydeammonia, acrylammonia,
hydrobenzamid, lophin, hydroanisamid, anisidin, hydrocuminamid,
hydrocinamid, besides waxes, and such biological substances as glucose,
lecithin, cholesterin, cholic, taurocholic, and glycocholic acids, and
cerebrin, all luminesce on oxidation. Radziszewski himself and many
other authors have compared the light of organisms to this type of
luminescence. Indeed the incorrect identification of granules found in
the cells of practically all luminous tissues as oil droplets, is
largely due to the influence of Radziszewski's work. Dubois (1901 _b_)
has added esculin, and Trautz (1904-5) many aldehydes and phenol
derivatives, including vanillin, papaverin, tannic and gallic acids,
besides glycerol and mannite to the list of biological substances
oxidizing with light production. Guinchant (1905) has described
oxyluminescence of uric acid and asparagine, Weitlaner (1911) of
substances in humus and McDermott (1913) of substances in urine and the
anaerobic alkaline hydrolysis products of glue and Witte's peptone.
Pyrogallol is especially prone to luminesce, as was first noticed by
Lenard and Wolf (1888) in developing a photographic plate with
pyrogallol developer. Later the luminescence was studied in some detail
by Trautz and Schloringin (1904-5) who developed the well-known
luminescent mixture of pyrogallol, formaldehyde, K_{2}CO_{3} and
H_{2}O_{2}. As I have shown, pyrogallol can be oxidized in a great many
different ways, and some of these are of great interest, for they very
closely imitate the mechanism for the production of light in organisms.
These are recorded in Table 3, which also includes various other types
of oxyluminescence of general or biological interest.
TABLE 3
_Types of Oxyluminescent Reactions_
1. Oxidation in air spontaneously.
(_a_) At ordinary temperatures. [Phosphorus. Fresh-cut surfaces of Na
or K. Thiophosgene and Thio-ethers (RCS.OR).]
(_b_) At melting or vaporizing points. (Fats, terpenes, sugars,
resins, gums, ether, silk and others.)
2. Oxidation in aqueous or alcoholic alkalies. (Many organic
substances.)
3. Oxidation in hypoiodites, hypobromites, or hypochlorites. (Many
organic substances.)
4. Oxidation in peroxides (H_{2}O_{2} or Na_{2}O_{2}). (Many organic
substances.)
5. Oxidation in ozone. (Many organic substances.)
6. Oxidation in acid permanganate. (Pyrogallol.)
7. Oxidation in persulfates and perborates. (Formaldehyde,
paraformaldehyde.)
8. Oxidation in perchlorates, periodates, and perbromates. (Palmitic
acid.)
9. Combination of 2 and 4. (Many organic substances.)
10. Combination of 3 and 4. (Many organic substances.)
11. Oxidation with H_{2}O_{2} and haemoglobin or vegetable oxidases.
(Pyrogallol, gallic acid, lophin, esculin.)
12. Oxidation with H_{2}O_{2} and MnO_{2}, Fe_{2}Fe(CN)_{6} Mn(OH)_{2} +
Mn(OH)_{3} Ag_{2}O, chromium oxide, cobalt oxide. (Pyrogallol.)
13. Oxidation with H_{2}O_{2} and ferrocyanides, chromates, bichromates,
permanganates, Fe salts, and Cr salts. (Pyrogallol, esculin.)
14. Oxidation with H_{2}O_{2} and collodial Ag. Pt. Pd. Au.
(Pyrogallol.)
The spectrum of chemiluminescent reactions has been described in a few
instances as continuous but no definite measurements of its extent have
been made. Radziszewski (1880) found the light of lophin oxidized in
alcoholic caustic alkali, examined with a two-prism spectroscope, to
give a continuous spectrum, brightest at _E_, with the red and violet
ends lacking. Trautz (1905, p. 101) states that the
pyrogallol-formaldehyde-Na_{2}CO_{3}-H_{2}O_{2} reaction gives a
continuous spectrum from the red to the blue green with maximum
brightness in the orange. Weiser (1918 _a_) has studied the spectra of
some chemiluminescent reactions by photographing the light behind a
series of color screens. He finds also that the spectra are short, with
maximum intensity in various regions. Thus, _amarin_ oxidized by
chlorine or bromine, extends from the yellow to greenish blue with a
maximum in the green while _phosphorus_, dissolved in glacial acetic
acid and oxidized with H_{2}O_{2}, luminesces from yellow green to
violet.
The spectra of luminous animals are quite similar to those of
chemiluminescent reactions. Moreover, as we have seen, chemiluminescence
is essentially an oxyluminescence, since oxygen is necessary for the
reaction. All luminous animals also require oxygen for light production.
Therefore, bioluminescence and chemiluminescence are similar phenomena
and they differ from all the other forms of luminescence which we have
considered. The light from luminous animals is due to the oxidation of
some substance produced in their cells, and when we can write the
structural formula of this photogenic substance and tell how the
oxidation proceeds, the problem of light production in animals will be
solved.
CHAPTER III
PHYSICAL NATURE OF ANIMAL LIGHT
Interest in the light of animals from a physical standpoint has centred
around questions of quality, efficiency and intensity, but in only one
group of luminous animals, the beetles, have accurate measurements of
these characteristics been made. This is due in part to the abundance of
these forms and their appeal to human interest and in part because they
are among the brightest of luminous organisms. Weak lights are not only
difficult to measure but, when dispersed to form spectra, give bands so
faint that their limits are very difficult to see and more so to
photograph. Very few organisms produce light visible to the fully
light-adapted eye. Although their light may seem quite bright to the
dark-adapted eye, the dark-adapted eye is a poor judge of the quality,
_i.e._, the color of a light. This is because of the Purkinje
phenomenon, a change in the region of maximum sensibility of the retina
with change in intensity of the light. For an equal energy spectrum, to
the normal, completely light-adapted eye, yellow-green light of
wave-length, [lambda] = .565 mu, appears the brightest, but when the light
is made fainter the maximum shifts first to the green and then to the
blue. The dark-adapted eye can see green or blue better than yellow and
for this reason weak lights will appear more green or blue than stronger
ones of the same energy distribution. Also two weak lights of the same
spectral composition may appear different in color if they differ much
in intensity. This is illustrated in Fig. 6.
[Illustration: FIG. 6.--Visibility curves for three illuminations
showing the shift in region of maximum visibility, or Purkinje
phenomenon (_after Nutting_).]
The shift in sensibility of the eye occurs in illuminations of between
0.5 and 50 metre-candles and represents a change from central cone
vision (high intensities) to peripheral rod vision (low intensities).
The _fovea centralis_ lacks rods and this part of the eye becomes
practically color blind at very low intensities of light. Below 0.5 and
above 50 metre-candles visibility varies but little with change in
intensity. It is clearly necessary then to distinguish between the
physical objective phenomenon of light and the physiological subjective
sensation of light.
It is a fact that different luminous animals produce light of quite
different colors as judged by our eye. A range of spectral tints has
been described which extends from red to violet but "yellowish,"
"greenish" and "bluish" tints are commonest. Indeed one or two animals
possess several luminous organs emitting lights of different colors.
This is true in a South American firefly, _Phengodes_, whose lights are
red and greenish yellow, and in the deep sea squid, _Thaumatolampas
diadema_, which produces lights of three colors, two shades of blue and
red. The red light in the case of the squid appears to be due to a red
color screen formed by the chromatophores, but in _Phengodes_ no screen
is present.
TABLE 4
_Wave-lengths of Fraunhofer Lines and Prominent Lines in Line Spectra_
FRAUNHOFER LINES
========================================================================
Line |Color | Wave-lengths | Source
| | ( mu mu = mu/1000) |
------------------+------+---------------------+------------------------
A |Red | 759.4 (band) |Oxygen in atmosphere.
a |Red | 718.5 (band) |Water vapor atmosphere.
B |Red | 686.7 |Oxygen vapor atmosphere.
C |Red | 656.3 |Hydrogen in sun.
D_{1} D_{2} |Yellow| 589.6, 589.0 |Sodium in sun.
E |Green | 527.0 |Calcium in sun.
b_{1} b_{2} b_{4} |Green | 518.4, 517.3, 516.8 |Magnesium in sun.
F |Blue | 486.1 |Hydrogen in sun.
G |Violet| 430.8 |Calcium in sun.
H K |Violet| 396.9, 393.4 |Calcium in sun.
------------------+------+---------------------+------------------------
BUNSEN FLAME LINES
===============================================
Source | Color | Wave-lengths ( mu mu = mu/1000)
----------+--------+---------------------------
Potassium | Red | 769.9, 766.5 (double)
Lithium | Red | 670.8
Sodium | Yellow | 589.6, 589.0 (double)
Thallium | Green | 535.1
Magnesium | Green | 518.4
Strontium | Blue | 460.7
----------+--------+---------------------------
PLUeCKER TUBE LINES
===============================================
Source | Color | Wave-lengths ( mu mu = mu/1000)
----------+--------+---------------------------
Mercury | Yellow | 579.0, 576.9
| Green | 546.1
| Blue | 491.6, 435.8
| Violet | 407.8, 404.7
Hydrogen | Red | 656.3
| Blue | 486.1, 434.1
Helium | Red | 728.2, 706.5, 667.8
| Yellow | 587.6
| Green | 504.8, 501.6, 492.2
| Blue | 471.3, 447.2
| Violet | 438.8, 402.6, 388.8
----------+--------+---------------------------
As we have seen, difference in color of the light does not necessarily
indicate difference in spectral composition because of the Purkinje
effect. However, examination of the spectrum of various luminous forms
has very clearly indicated that the different colors are really due to
light rays of different wave-length and are not the result of any
subjective phenomena. To facilitate comparison, spectral lines and
colors are given in Table 4. The first adequate observations on the
spectra of luminous animals were made by Pasteur (1864), who studied
_Pyrophorus_ and found a continuous spectrum unbroken by light or dark
bands. Lankester (1868) discovered a similar continuous spectrum in
_Chaetopterus insignis_ and placed its limits from line 5 to 10 on
Sorby's Scale (about [lambda] = 0.55 mu to [lambda] = 0.44 mu). Young
(1870) first recorded the limits of the firefly spectrum as a little
above _C_ ([lambda] = .6563 mu) to _F_ ([lambda] = .4861 mu). Since then a
number of luminous forms have been examined and all are found to give
short continuous spectra (not crossed by light or dark bands or lines)
lying in different color regions. Thus, Conroy (1882) examined the
glowworm (_Lampyris noctiluca_) light and observed a band extending from
[lambda] = 0.518 mu to [lambda] = 0.656 mu. Dubois (1886) states that the
spectrum of _Pyrophorus noctilucus_, the West Indian "Cucullo," extends
from slightly further than the Fraunhofer _B_ line to the _F_ line,
while Langley and Very (1890), working on the same form, placed the
limits at [lambda] = 0.468 mu to [lambda] = 0.640 mu. It consists, then, of
a broad band chiefly in the green and yellow. But, "would the light not
extend farther were it bright enough to be seen?... if the light of the
insect were as bright as that of the sun would it not extend equally far
on either side of the spectrum?" "It is impossible to increase the
intrinsic brilliancy by any optical device, but if it be impossible to
make the light of the insect as bright as that of the sun, it is on the
other hand quite possible to make the light of the sun no brighter than
that of the insect ..." Langley and Very investigated this question,
forming a solar spectrum from sunlight of the same intensity as that of
_Pyrophorus_ and a _Pyrophorus_ spectrum together in the same field of
the spectroscope. The latter was very much shorter than the solar
spectrum, showing that its length was not due to weakness of the red and
blue rays but to their absence. Later Ives and Coblentz (1910)
photographed the spectrum of a firefly (_Photinus pyralis_), together
with that of a carbon glow lamp, on plates sensitive to all wave-lengths
of visible rays under conditions which would have recorded all visible
radiations given off. They found the spectrum to extend only from
[lambda] = 0.51 mu to [lambda] = 0.67 mu (Fig. 7). Another species of
firefly (_Photuris pennsylvanica_) was found by Coblentz (1912) to give
a spectrum extending from [lambda] = 0.51 mu to [lambda] = 0.59 mu (Fig. 8).
The _Photinus_ light extends much further into the red and it is easy to
distinguish between _Photinus_ and _Photuris_ in nature, merely by the
reddish tint of the light of the former. These photographic records show
conclusively that the color of the light of luminous animals is not a
subjective phenomenon due to the Purkinje effect and the low intensity
of the light, but is real, an actual difference in spectral
composition of the light emitted. Neither is it due, at least in the
fireflies examined, to the existence of color screens which absorb
certain rays, allowing only those of a definite color to pass. The
spectra of forms thus far investigated are reproduced in Fig. 9 and
recorded in Table 5. It will be noted that they vary considerably in
position but are all of the same type. The spectrum of _Cypridina
hilgendorfii_ is the longest thus far investigated ([lambda] = .610 mu to
[lambda] = .415 mu), extending well into the blue, and the light of this
form is very blue in appearance.
[Illustration: FIG. 7.--Spectra of carbon glow lamp, A, firefly
(_Photinus pyralis_); B, and helium vacuum tube, C (_after Ives and
Coblentz_).]
[Illustration: FIG. 8.--Spectra of helium vacuum tube (1); carbon glow
lamp (2); the firefly, _Photinus pyralis_ (3); and the firefly _Photuris
pennsylvanica_ (4) (_after Coblentz_).]
[Illustration: FIG. 9.--Spectra of various luminous animals (_after
McDermott_). 1. Portion of the visible solar (grating) spectrum showing
Fraunhofer lines. 2. _Pyrophorus noctilucus_ (Langley and Very.) 3.
_Lampyris noctiluca_ (Conroy). 4. _Photinus pyralis_ (Ives and
Coblentz). 5. _Photinus consanguineus_ (Coblentz). 6. _Photuris
pennsylvanica_ (Coblentz). 7. _Phengodes laticollis_ (McDermott). 8.
_Bacterium phosphoreum, B. phosphorescens or Bacillus photogenus_
(Molish). 9. _Photobacterium indicum_ (Barnard). 10. _Mycelium X_
(Molish). 11. _Luminous bacteria_ (Foerster). 12. _Agaricus sp._?
(Ludwig). 13. Fluorescent spectrum of luciferesceine of _Photinus
pyralis_ (Coblentz). Only the extreme ends of the bands are shown and no
attempt is made to indicate the relative density of different portions
of the spectra.]
TABLE 5.--_Limits of Spectra of Various Luminous Organisms_
============================================================================
Light | Spectrum ( mu) |Emission |Observer |Method and remarks
| |maximum | |
----------------+----------------+---------+-----------+--------------------
Cypridina | 0.610-0.415 | |Harvey, |Eye observation,
hilgendorfii | | | 1919 | Zeiss comparison
| | | | spectroscope.
Chaetopterus | 0.55-0.44 | |Lancaster, |Eye observation.
insignis | (approximately)| | 1868 |
Pyrophorus | 0.72-0.486 | |Dubois, |Eye observation.
noctilucus | | | 1886 |
Pyrophorus | .640 - .468 | 0.57 |Langley |Eye observation
noctilucus | | | and Very,| and comparison
(thoracic | | | 1890 | with solar
light) | | | | spectrum of
| | | | equal intensity.
Pyrophorus | .663 - .463 | | |
noctilucus | | | |
(abdominal | | | |
light) | | | |
Photinus | .67 - .51 | |Ives and |Photographic
pyralis | | | Coblentz,| comparison with
| | | 1909 | carbon glow lamp
| | | | of equal
| | | | intensity.
Photuris | .59 - .51 | .552 |Coblentz, |Photographic
pennsylvanica | | | 1912 | comparison with
| | | | carbon glow lamp
| | | | of equal
| | | | intensity.
Photinus | .65 - .52 | .578 |Coblentz, |Photographic
consanguineus | | | 1912 | comparison with
| | | | carbon glow lamp
| | | | of equal
| | | | intensity.
Phengodes | .65 - .52 | |McDermott, |Eye observation.
laticollis | | | 1911 e |
Lampyris | .656- .518 | |Conroy, |Eye observation.
(glow worm) | | | 1910 |
Photinus | .670- .487 | |Young, |Eye observation
| | | 1870 | direct vision
| | | | spectroscope.
Bacteria |G to F extending| |Barnard, | Photographic.
| toward D for | | 1902 |
| long exposure | | |
Bacteria |Somewhat beyond | |Fisher, |Eye observation.
| G to D | | 1888 |
Bacteria | .58 - .43 | |Foerster, |Eye observation
| | | 1887 | Zeiss. Abbe
| | | | microspectral
| | | | ocular.
Bacteria | >.500 to .350 | Bright |Forsyth, |Photographic,
| | band | 1910 | quartz
| | at .4 | | spectroscope.
Agarious | 0.56-0.48 | |Ludwig, |Eye observation,
melleus | (approximately)| | 1884 | Sorby Brown
| | | | microspectroscope.
Xylaria | .54 - .46 | |Ludwig, |Eye observation,
hypoxylon | (approximately)| | 1884 | Sorby Brown
| | | | microspectroscope.
Micrococcus |b into the | |Ludwig, |Eye observation,
Pflugeri | violet | | 1884 | Sorby Brown
| | | | microspectroscope.
Mycelium X | .570 - .480 | |Molish, |Eye observation,
| | | 1904, | Zeiss comparison
| | | book | spectroscope.
Bacterium | .570 - .450 | |Molish, |Eye observation,
phosphoreum | | | 1904, | Zeiss comparison
| | | book | spectroscope.
Bacterium | .570 - .450 | |Molish, |Eye observation,
phosphorescens| | | 1904, | Zeiss comparison
| | | book | spectroscope.
Bacillus | .570 - .450 | |Molish, |Eye observation,
photogenes | | | 1904, | Zeiss comparison
| | | book | spectroscope.
Pseudomonas | .570 - .450 | |Molish, |Eye observation,
lucifera | | | 1904, | Zeiss comparison
| | | book | spectroscope.
----------------+----------------+---------+-----------+--------------------
As first shown by Dubois (1886) for _Pyrophorus_, and confirmed by
myself for _Cypridina_, the light is not polarized in any way. I may add
that the _Cypridina_ light like any other light may be polarized by
passing through a Nicol prism.
Several writers [Dubois (1914 book)], Fischer (1888), Molisch (1904
book) have noticed that the light of luminous bacteria changes in color
if grown on different culture media. Light which is "silver white" on
dead fish becomes "greenish" on salt-peptone-gelatin media and more
yellow on salt-poor media. Peron (1804) and Panceri (1872) describe the
light of _Pyrosoma_ as yellow to greenish after death of the animal and
reddish on stimulation; then fading out through orange, yellow, greenish
and azure blue. Polimanti (1911) describes the normal light of
_Pyrosoma_ as greenish, and states that as the animals die, or if they
are kept at temperatures above the optimum, the light becomes more red.
McDermott (1911, _b_) noticed that the light of fireflies placed in
liquid air became decidedly reddish just before going out and on
rewarming the first light to appear was reddish followed by the proper
shade at higher temperatures. I have frequently observed a more
reddish color from luminous tissues of the firefly upon the addition of
coagulants such as alcohol, and have noted that the light of _Cypridina_
becomes weaker and more yellow at both low (0 deg.) and high (50 deg.)
temperatures. The meaning of these color changes will be discussed in
Chapter VII.
The efficiency of any light may be defined in several different ways:
(1) By the percentage of visible wave-lengths in the total amount of
radiation emitted, _i.e._, visible radiation divided by total (heat,
visible, actinic) radiation; (2) by considering, in addition to visible
radiation / total radiation, the sensibility of the eye to different
wave-lengths, visible radiation x visual sensibility / total radiation.
Visible radiation x visual sensibility is spoken of as luminosity; (3)
by the amount of light (expressed in candles) produced in relation to a
given expenditure of energy or in relation to the cost of the energy
expended. Thus, of the radiation emitted from an incandescent electric
lamp only a small per cent. is light, the rest being heat and actinic
rays. It is therefore very far from being 100 per cent. efficient. If
there were no infra-red or ultra-violet in the radiation from an
incandescent lamp its efficiency would be 100 per cent. if we
disregarded visual sensibility. But if we take into account the fact
that the eye is most sensitive to yellow green, a source of light, even
though emitting only visible radiation, would not be 100 per cent.
efficient unless its maximum of emission corresponded also with the
maximum of visual sensibility. We shall return to this question in a
later paragraph. Looking at the question from the standpoint of energy
consumption, the carbon incandescent lamp gives one mean spherical
candle for 4.83 watts (watt = 10^7 ergs per sec.), while the tungsten
lamp gives one mean spherical candle for 1.6 watts, about one-third the
energy, and the latter is consequently more efficient.
As we know practically nothing of the energy transformations occurring
during the process of light production in organisms, all statements
regarding the efficiency of their light are based on relations between
the visible radiation and total radiation. This involves a measurement
of rays in the infra-red region (heat rays) and ultra-violet region
(actinic rays) as well as the light rays proper, and any other radiant
energy produced. While all spectroscopic investigations show that the
spectrum of luminous animals never extends to the limits of the visible
spectrum in either the red or violet, it is possible that bands occur in
the infra-red or ultra-violet, and special methods must be employed to
detect these. Radiations of all kinds, if converted into heat on
striking the blackened surface of a thermopile, bolometer, or radiometer
can be measured by changes in temperature and the relative amounts of
energy represented be compared in a common unit, the calorie. By proper
screening, all rays except the visible light rays can be cut off from
the measuring instrument and the amounts of energy represented in light
and in total radiation thus be determined.
Dubois (1886) first studied this problem in _Pyrophorus_ by the use of a
thermopile and galvanometer and found a small amount of radiation from
the luminous region in excess of that from a non-luminous region. It
amounted to a galvanometer deflection of 0.95 deg. and was increased 0.3 deg.
during the flash of the insect on electrical stimulation. This increase
of 0.3 deg. is possibly due to heat produced on muscular contraction. In any
case the amount of heat radiated in comparison with that of the candle
is very small indeed. A more careful study has been made by Langley and
Very (1890) with the bolometer. They point out first of all that the
total radiation from the most powerful luminous organ (the abdominal
one) of _Pyrophorus_ which affected their bolometer slightly, would, in
the same time (10 seconds), be sufficient to raise the temperature of an
ordinary mercurial thermometer having a bulb 1 cm. in diameter by rather
less than 2.3 x 10^{-6} deg. C. We may thus gain some idea of the magnitude
of the measurements to be made. The radiation from _Pyrophorus_ which
affected their bolometer was shown to be due merely to the "body
heat"[2] of the insect, and it is largely cut off by a plate of glass
which is opaque to all wave-lengths of 3 mu or more. These waves are given
off by bodies at temperatures below 50 deg. C. and belong "to quite another
spectral region to that in which the invisible heat associated with
light mainly appears." Langley and Very then compared the radiation from
a non-luminous bunsen flame and the _Pyrophorus_ light, interposing a
plate of glass in each case to cut off the waves longer than 3 mu, and
found several hundred times more radiation in the case of the bunsen
burner but, nevertheless, perceptible radiation from _Pyrophorus_. The
former consisted of radiant heat shorter than [lambda] = 3 mu and
extending up to the visible light rays ([lambda] = 0.7 mu since the bunsen
flame emitted no light). The very slight effect of the _Pyrophorus_
radiation must be due to wave-lengths between [lambda] = 3 mu and [lambda]
= 0.468 mu, the limit of the _Pyrophorus_ spectrum in the blue. Langley
and Very assumed it to be due entirely to the band of visible light,
[lambda] = 0.640 mu to [lambda] = 0.468 mu, and assumed that no invisible
heat rays were produced. All of the energy of _Pyrophorus_ light would
therefore lie in the visible region and its efficiency (light rays /
heat + light + actinic rays) would be 100 per cent. Later, Langley
(1902) reinvestigated the radiation of _Pyrophorus_ and could detect no
heating whatever with the bolometer. "A portion of the flame of a
standard sperm candle, equal in area to the bright part of the insects,
gave under the same circumstances, a bolometric effect of such magnitude
that had the heat of the insect been 1/80,000 as great as that from the
candle, it would certainly have been recognized." Coblentz (1912) also,
using a vacuum thermopile of Pt and Bi, was unable to detect any
infra-red radiation from _Photinus pyralis_, but found that the
temperature of this firefly is slightly lower than the air. These
temperature measurements will be discussed in a later chapter.
[2] Langley and Very evidently supposed that the body temperature of the
firefly, like the mammal or bird, is higher than its surroundings.
The assumption of Langley and Very that the small amount of _Pyrophorus_
radiation passing glass is all light has been called into question by
Ives (1910), who points out that Langley and Very failed to use a screen
which would cut off either the visible rays or the invisible rays
between 3 mu and 0.7 mu. They really left the question open as to whether
the effect of _Pyrophorus_ light on their bolometer was due to the
visible band of rays or to this plus another band in the infra-red. "The
firefly's actual efficiency as a light source is dependent to a large
degree on the radiation being confined to the visible region. If there
should be found infra-red of quantity comparable to the visible, the
firefly, while still a very efficient source would not be, as usually
supposed, the example of an ideally efficient light produced by
nature."
Ives investigated the question further by the phosphor-photographic
method. "In brief it consists of this: Phosphorescence, which is excited
in various substances by exposure to short waves (blue, violet or
ultra-violet), is destroyed by exposure to longer waves (orange, red,
infra-red). Thus, a surface of Balmain's paint or of Sidot blende,
excited to phosphorescence and then exposed in a spectrograph, will have
areas of reduced brightness wherever long-wave energy has fallen upon
it. If this surface is then laid on a photographic plate for a short
period, a permanent record is obtained on the plate after development."
Preliminary tests showed that the method was applicable in the case of
weak light such as the firefly spectrum and also if the light is
intermittent like the firefly. With Sidot blend (ZnS) the extinguishing
action extends from [lambda] = 0.6 mu to [lambda] = 1.5 mu. A sheet of deep
ruby glass, which cut off all the visible rays of the firefly but
allowed infra-red to pass, was placed between the firefly light and a
surface of phosphorescent Sidot blend which was exposed to the firefly
flashes for three and a half hours. No extinction of phosphorescence
occurred, while without the ruby glass, extinction, due to the orange
rays of the _visible_ firefly light was noticeable in 20 minutes. There
is thus no infra-red of an intensity at all comparable to the visible as
far as [lambda] = 1.5 mu, the lower limit of the phosphor-photographic
method. Coblentz (1912) had examined the transparency of the dry
chitinous integument of various fireflies (Fig. 10) in the infra-red and
reports it to be fairly transparent down to [lambda] = 2.8 mu, opaque
between [lambda] = 2.8 mu and [lambda] = 3.8 mu, transparent again to
[lambda] = 6 mu, and opaque beyond that. The infra-red could, then, if it
were emitted, largely pass through the integument which is similar in
absorption properties to complex carbohydrates. Transparency of the
integument to the ultra-violet was not studied.
[Illustration: 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 [lambda] = 0.50 mu (the lower limit
of sensitivity of the plate) to [lambda] = 0.35 mu. 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 glass 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 passes quartz but not glass, 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 [lambda] = 0.216 mu.
It will thus be seen that the radiation from the firefly has been very
carefully studied and that no waves are given off from [lambda] = 1.5 mu
to [lambda] = 0.216 mu with the exception of the short band ([lambda] =
0.67 mu to [lambda] = 0.51 mu) in the visible, and it is highly probable
that no radiation is given off with wave-lengths longer than [lambda] =
1.5 mu. 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
title 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."
[Illustration: 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 [lambda] = 0.565 mu,
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 [lambda] = 0.565 mu 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_ x
_visual sensibility_) or luminosity / total radiated energy.
[Illustration: FIG. 12.--Visibility curves of various investigators
obtained by different methods (_after Hyde, Forsyth and Cady_).]
[Illustration: FIG. 13.--Luminous efficiency of the 4-watt carbon glow
lamp, shaded area / total area (_after Ives and Coblentz_).]
[Illustration: 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 illustration 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_.
[Illustration: 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 deg. 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 [lambda] = .590 mu, 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 x
| | watt |visual
| | |sensibility /
| | |total radiation)
------------------------+------------------------+--------+----------------
Carbon incandescent lamp| 4 watts per mean | 2.6 | 0.0042
oval anchored (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
| | |
Glass 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 ([lambda] = 0.567 mu) does not
correspond with the maximum sensibility of the eye ([lambda] = 0.565 mu),
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 relationships, 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 pass 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 pass 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 deg.-35 deg. 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 glass.
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 glass 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 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/_r_^2) by a point source of one candle-power. One
candle-power emits 4[pi] lumens. The latest figure for the mechanical
equivalent of light at [lambda] = .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 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 x 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 x
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 x 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 x
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.
We know of no animal whose eyes, the organs, _par excellence_, of
photochemical change, give off light in the dark. All cases of luminous
eyes have been conclusively shown to be purely reflection phenomena. The
eyes of a cat only glow if some stray light is present which may enter
and be reflected out again. Photochemical reactions and chemiluminescent
reactions do have this in common, however, that they are largely but not
exclusively oxidations. Whether all photochemical changes in the eyes in
animals require oxygen or not, is unknown, but all animal
light-producing reactions, without exception, are oxidations, and light
is only produced if oxygen is present. Some material is oxidized.
In general, we may divide luminous organisms into two great classes
according as the oxidizable material is burned within the cell where it
is formed or is secreted to the exterior and is burned
outside--intracellular and extracellular luminescence. Many animals with
intracellular luminescence have quite complicated luminous organs. It is
an interesting fact that a great similarity may be observed between the
evolution of the complex organs of vision and of these complicated
organs. In the simplest unicellular forms certain structures within the
cell serve as the photochemical detectors of light, while in luminous
protozoa, similarly, granules scattered throughout the cell are oxidized
with light production. In the higher forms the eye contains groups of
photosensitive cells connected with afferent nerves, lenses, and
accessory structures for properly adjusting the light, while luminous
organs contain groups of photogenic cells in connection with efferent
nerves, lenses, and accessory structures for properly directing the
light. It is interesting to note that in the two groups where the eye
has attained its highest development, the cephalopods and vertebrates,
here also the luminous organ is found in greatest complexity and
perfection. In intermediate stages of evolution the eye and luminous
organ so closely approach each other in structure that it is still a
mooted question whether certain organs found in worms and crustacea are
intended for receiving or producing light.
We may also divide luminous forms into two groups according as the
oxidation of luminous material goes on continuously, independently of
any stimulation of the organism; or is intermittent, oxidation and
luminescence occurring only as a result of stimulation, using the word
"stimulation" in the same sense in which it is used in connection with
nerve or muscle tissue. Bacteria, fungi, and a few fish produce light
continuously and independently of stimulation. Its intensity varies only
over long periods of time and is dependent on the nature of the nutrient
medium or general physiological condition of the organism. All other
forms give off no light until they are stimulated. Stimulation may of
course come from the inside (nerves) or outside. Only under unfavorable
conditions, such as will eventually lead to the destruction of the
luminous cells, do these forms give off a continuous light. This has
often been spoken of as the "death glow," and is to be compared with
_rigor_ in muscle tissue.
Some of the fish which produce a continuous light possess a movable
screen similar to an eyelid which can be drawn across the organ, thus
shutting off the light, so that the animal appears to belong to the
group which flashes on stimulation. This is true of _Photoblepharon_,
while _Anomalops_ can rotate the light organ itself downward, so as to
bring the lighting surface against the body wall and thus cut off the
light (Steche, 1909). Other fish (_Monocentris_) are unable to "turn
off" their light.
Animals which flash spontaneously on stimulation through nerves from
within, possess a very varied rhythm. The different species of fireflies
can be distinguished by the character of their flashing (McDermott,
1910-17; Mast, 1912). Fig. 16 shows the method of flashing of some
common eastern North America species. The glowworm light lasts for many
seconds and then dies out. This interval of darkness persists for some
minutes and is then followed by another period of glowing. Some
fireflies have a light which may be described as partially
intermittent. It lasts for hours, but may become more dim or be
intensified on stimulation.
[Illustration: FIG. 16.--Chart showing relative intensities and
durations of flashes of American fireflies (_after McDermott_). One cm.
vertically = approximately 0.02 candle power; one cm. horizontally =
approximately one second. The flash of the males ([male]) is at the
left; that of females ([female]) at right of chart.]
Some forms only produce light at certain seasons of the year. According
to Giesbrecht (1895) this is true of the copepods, which only light in
summer and autumn, and according to Greene (1899) in the toad-fish;
_Porichthys_, which can only be stimulated to luminesce during the
spawning season in spring and early summer.
Some animals possess a periodicity of luminescence. They only luminesce
at night and fail to respond to stimulation or are difficult to
stimulate during the day. Bright light has an inhibiting effect. Perhaps
correlated with this is the fact that most luminous forms are strongly
negatively heliotropic. Fireflies lie hidden in the day, to appear about
dusk and the ostracod crustacean, _Cypridina_, is difficult to obtain on
moonlight nights.
The Ctenophores were the first forms in which the inhibiting effect of
light was noticed. This was described by Allman (1862) and has been
confirmed by a number of observers, especially Peters (1905). Massart
found that _Noctiluca_ was difficult to stimulate during the day and
_Ceratium_, according to both Zacharias (1905) and Moore (1908), only
luminesces at night, or if kept in darkness, for some little time.
Crozier[4] finds a persistent day-night rhythm of light production when
_Ptychodera_, a balanoglossid, is maintained for eight days in continued
darkness. The animal is difficult to stimulate during the period which
corresponds to day and luminesces brilliantly and at the slightest touch
during the period which corresponds to night.
On the other hand, a great many forms are able to luminesce quite
independently of previous illumination. According to Crozier[4]
_Chaetopterus_ luminescence is not affected by an exposure to 3000
metre-candles for six hours.
[4] Private communication.
In the case of animals with extracellular luminescence we may speak of
luminous secretions and true luminous glands. A large number of forms
possess luminous glands or gland cells, including some of the _medusae_,
the hydroids (probably), the pennatulids (?), the molluscs (_Pholas_ and
_Phyllirhoe_) (probably), some cephalopods (_Heteroteuthis_ and
_Sepietta_), most annelids, ostracods, copepods, some schizopods
(_Gnathophausia_) and decapod (_Heterocarpus_ and _Aristeus_)
crustaceans, all myriapods, and the balanoglossids. The remaining
organisms burn their material within the cell. These include the
bacteria, fungi, protozoa, some medusae (?), ctenophores (probably), most
cephalopods, a few annelids (_Tomopterus_ (?)), ophiuroids (?), some
schizopod (_Nyctiphanes_, _Euphasia_, _Nematocelis_, _Stylochiron_) and
decapod (_Sergestes_) crustacea, all(?) insects, _Pyrosoma_, and fishes
(_selachians_ and _teleosts_). It is among this latter type that the
most complicated luminous organs have been developed. While a
description of all the types of luminous organs and luminous structures
cannot be attempted here (excellent descriptions have been given by
Dahlgren and Mangold) it is necessary to understand the structural
conditions in a few of the forms whose physiology has attracted most
attention.
Luminous bacteria are so small that the light from a single individual
cannot be seen. It is almost impossible to make out structural
differences within the cell and we cannot definitely state in just what
special region, if any, the luminescence is produced. We do know that
the light is intracellular and that filtration of the bacteria from
their culture medium gives a dark sterile filtrate absolutely free from
any luminous secretion.
Among protozoa, in certain forms at least, it is easy to observe that
luminescence is connected with globules or granules which were
considered by the earlier observers to be oil droplets. Thus, in
_Noctiluca_ (Figs. 17 and 18), when the animal is violently stimulated
or in the presence of reagents which slowly kill it, the whole interior
appears a mass of starry points of light which can be traced to minute
granules along the strands of protoplasm (Quatrefages, 1850).
[Illustration: FIG. 17.--_Noctiluca miliaris_, showing photogenic
granules in cytoplasm. _n_, nucleus; _c_, cytoplasmic strands containing
photogenic (large) and other (small) granules; _p_, pharynx; _f_,
flagellum; _o_, oral groove; _t_, tentacle; _s_, spines at base of
tentacle; _v_, vacuoles. _Drawn by E. B. Harvey._]
[Illustration: FIG. 18.--_Noctiluca miliaris_ as it appears during
luminescence (_after Quatrefages_). Upper left and middle, low power;
below, high power; upper right, a crushed fragment still luminescent.]
Turning to the multicellular forms, we find the simplest development of
luminosity in those animals which possess gland cells producing a
luminous secretion. These cells may be scattered over the surface of the
animal as in _Chaetopterus_ (Fig. 19) or _Cavernularia_, or restricted to
certain areas [_Pholas_, (Fig. 19),] or more definitely localized to
form an isolated group of gland cells as in _Cypridina_. True
multicellular glands also occur. In every case, however, we find that
the luminosity of these uni- or multicellular glands is connected with
the presence of granules. They are often spoken of as _luciferine
granules_, although it is not certain whether they are made up of
luciferin or luciferase (see Chapter IV) or both. They are most similar
to the zymogen granules found so abundantly in gland cells and thought
to be the precursors of various enzymes. According to Dahlgren (1915),
the luciferine granules stain blue-black by iron haematoxylon after
fixation at the boiling point, and photogenic cells can be detected by
this method of selective staining. Dubois (1914, book), who regards them
as examples of _bioprotein_, comparable to the chondriosomes and handed
on from one generation to another, gives them the name of _vacuolides_
or _macrozymases_. In some forms he has described their transformation
into crystals and believed at one time that animal light was a
crystalloluminescence. His figures of the crystal transformation are
not very convincing. Pierantoni (1915) has considered the granules to be
_symbiotic_ luminous bacteria, but this is certainly not the case.
[Illustration: FIG. 19.--Diagram of _Pholas_ (right) and _Chaetopterus_
(left) to show distribution of luminous areas (_after Panceri_).]
The light of _Chaetopterus_ comes from a material mixed with a mucous
secretion formed over almost the whole body surfaces of the animal. A
section of the epithelium shows large mucous-producing cells and smaller
granule-containing light cells (Fig. 20). These appear to be under
nervous control, as a strong stimulation in one part of the body causes
luminescence which spreads over the whole surface of the worm. The
animal becomes fatigued rather readily, however. In the pennatulids,
such as _Cavernularia_, we have also the formation of a luminous
secretion over the whole surface of the body and the individual animals
in this colonial form are also connected with nerves. A stimulation in
any local region, as Panceri (1872) first showed (Fig. 21), will cause a
wave of luminosity to spread from this point until it extends over the
whole surface of the colony. In _Pennatula_ the rate of this luminous
wave is about 5 cm. per second.
[Illustration: FIG. 20.--Sectional view of the luminous epithelium of
_Chaetopterus_ (_after Dahlgren_). _cu_, cuticle; _l. c._, light cells,
some showing discharge of secretion; _d. l. c._, discharged and emptied
light cells; _m. c._, mucous cells.]
[Illustration: FIG. 21.--Diagram of _Pennatula_, showing by arrows the
course of a wave of luminosity which spreads over the colony from the
point stimulated (s) (_after Panceri_).]
_Pholas dactylus_ possesses similar light cells to those of
_Chaetopterus_, but they are restricted to narrow bands on the siphon and
mantle and a pair of triangular spots near the retractor muscles. Nerves
pass to the luminous regions.
In many luminous animals the light secretion formed over the surface of
the body is small in amount and adheres to the animal because it is
embedded in the mucous skin secretions. In those forms which possess a
true localized light gland the luminous secretion when expelled into the
sea water (if the animal be a marine form) may persist as a luminous
streak for some time and exhibit diffusion and convection
movements. The most beautiful examples of luminous secretions are found
among the ostracod crustacea.
[Illustration: FIG. 22.--Luminous gland of _Cypridina hilgendorfii_
(_after Yatsu_). 2, longitudinal section. 4, transverse section.]
[Illustration: FIG. 23.--Single enlarged gland cell of _Cypridina_
(_after Dahlgren_). P, nucleus and plasmasome; C, cytoplasm; F,
secretion fibrils; D, reservoir duct filled with large yellow granules;
O, valve-like outer opening of cell at surface of body.]
In _Cypridina hilgendorfii_ the luminous gland is situated on the upper
lip near the mouth. It is made up of elongate (some 0.7 mm. in length),
spindle-shaped cells, each one of which opens by a separate pore with a
kind of valve. The openings are arranged on five protuberances. Muscle
fibres pass between the gland cells in such a way that by contracting
the secretion can be forced out. In the sea water the secretion
luminesces brilliantly and the Japanese call these forms _umi hotaru_,
or marine fireflies. Fig. 22 is a diagram showing the structure.
Watanabe (1897), who first studied this form, and also Yatsu (1917) have
described two kinds of granule-containing cells, one with large yellow
globules, 4-10 mu in diameter (Fig. 23), the other with small colorless
granules 0.5, in diameter. I have observed in the living form these two
types and also large colorless globules of the same size as the yellow
globules. All dissolve when extruded into the sea water. Dahlgren[5] has
described from sections four types of cells containing (1) large
globules, (2) small granules, (3) a fat-like material, (4) a mucous
material. Just what the significance and nature of these types of
substance is cannot be stated at present. At least one, probably two,
are concerned in light production. The others may possibly form
digestive fluids which act on the food of the animal.
[5] Private communication soon to be published.
Turning now to the animals possessing light cells with intracellular
luminescence we find in general that such light cells are localized to
form definite light organs and that these may be single, as in the
common fireflies, paired, as the prothoracic light organs of
_Pyrophorus_, or scattered over the surface of the body, as in so many
shrimps, cephalopods and fishes, when they are often called photophores.
The light cells proper are often associated with reflectors, lenses,
opaque screens and color screens.
[Illustration: FIG. 24.--Distal portion of malpighian tubule of
_Bolitophila_, showing modification to form photogenic organ (_after
Wheeler and Williams_). _MT_{1}_, _MT_{4}_, malpighian tubules forming
photogenic organ; _R_, reflector; _M_, muscle; _T_, trachea.]
The insects possess the simplest types of intracellular light organs, a
mass of photogenic cells, which, in the common firefly (_a lampyrid
beetle_) of Eastern North America, has probably been developed from the
fat body, while in the New Zealand glowworm, the larva of a tipulid fly
(_Bolitophila luminosa_), part of the Malpighian tubule cells have
acquired photogenic power (Wheeler and Williams, 1915). This is
illustrated in Fig. 24.
The photogenic organ of the firefly is made up of two kinds of cells, a
dorsal mass of small cells several layers deep, the reflector layer, and
a ventral mass of large cells with indistinct boundaries, the photogenic
layer (Fig. 25). The photogenic cells contain a mass of granules,
spherical in the male and short rods in the female. The photogenic cells
are divided into groups by large tracheal trunks which pass into the
light organ and branch to form tracheoles connected with tracheal end
cells. The exact distribution varies in different species, but in all
the arrangement is such as to give a very abundant oxygen supply. Each
group of photogenic cells is surrounded by a clear ectoplasm containing
no granules. The tracheoles pass through this and either end openly
within the photogenic cells or anastomose with tracheoles from
neighboring tracheae. Nerves, but no blood-vessels--which are absent in
insects--enter the organ. It is difficult to determine if the nerves
supply the tracheal end cells or the photogenic cells.
[Illustration: FIG. 25.--Sectional view of photogenic organ of the
firefly (_after Williams_), showing reflector or crystal layer (_U_)
above and photogenic cells (_P_) below. _C_, cuticula; _T_, trachea;
_c_, capillaries of tracheal end cells; _H_, hypodermis; _EC_, tracheal
end cells; _N_, nerve.]
The dorsal reflecting layer is made up of cells containing numerous
minute crystals of some purin base, either xanthin or urates, or both.
They have a white milky appearance and while they are certainly not good
reflectors in the optical sense, they do act as a white background,
scatter incident light, and partially prevent its penetration to the
internal organs of the firefly. Although a few crystals similar to those
of the reflector layer are found in the photogenic cells and in other
cells of the body, it is known that the photogenic cells are not
transformed into the reflector cells. The two layers are distinct and
permanent from an early stage in development.
Curiously enough, the light organ of the larva of the firefly (glowworm)
is quite distinct from that of the adult. Like so many other structures
in insects, the adult organ is developed anew from potential photogenic
cells during the pupal period. Even the egg of the firefly is luminous
and glows with a steady light, and during the pupal period light may
sometimes be seen coming from the thoracic region.
In the firefly there is no true lens, the light merely shining through
the cuticle which is transparent over the light organ, whereas over the
rest of the body it is dark and pigmented. In the deep sea shrimp,
_Acanthephyra debelis_, with light organs scattered over the surface of
the body, the cuticle covering the light organ forms a concavo-convex
lens, behind which are the photogenic cells (Kemp, 1910). As may be seen
from Fig. 26, the lens is made up of three layers which suggests that it
may be corrected for chromatic aberration--a veritable "achromatic
triplet." In an allied form, _Sergestes_ (Fig. 27), the lens is of two
layers and double convex. Optical studies of these lanterns have been
made by Trojan (1907). The course of the light rays is shown in Fig. 28.
The lens of these organs is also bluish in color which suggests that
they may serve also as color filters. Behind the photogenic cells is a
mass of connective tissues through which enters the nerve, for the
light of these organs is under the control of the animal and may be
flashed "at will."
[Illustration: FIG. 26.--Sectional view of photogenic organ of
_Acanthephyra debilis_ (_after Kemp_). _n_, nerve; _s. l._, sheathing
layer of cells; _g_, cone of refractive granules at end of nerve strand;
_c_, cellular layer; _i. l._, _m. l._, _o. l._, inner, middle and outer
layer of lens.]
[Illustration: FIG. 27.--Sectional view of photogenic organ of
_Sergestes prehensilis_ (_after Terao_). _bm_, basement membrane; _cs_,
connective strands of photogenic layer; _hy_, hypodermis; _l_{1}_,
_l_{2}_, _l_{3}_, layers of lens; _le_, lens epithelium; _n_, nerve;
_ph_, photogenic cells; _pi_, pigment layer; _r_, reflector; _th_,
theca.]
[Illustration: FIG. 28.--Diagram of photogenic organ of _Nyctiphanes
Conchii_, to show pathways of light rays arising in the light cell layer
(_after Trojan_). _p_, pigment; _ri_, inner reflector; _lp_, light
cells; _rf_, refractor; _f_, focus; _l_, lens; _A-A_, axis;
_a_{1}-a_{4}_, _b_{1}-b_{4}_, light rays reflected from _ri_;
_c_{1}-c_{4}_, light rays passing directly outward; _d_{1}-d_{9}_ and
_e_{1}-e_{9}_, light rays which have passed refractor and lens
respectively.]
All gradations in complexity of light organs may be found from the
condition in the shrimp just described to that found among the squid and
fish. Figs. 29 and 30 are sections of two of the more complicated types
found in squid. The explanation given to the various structures is that
of Chun (1903) to whom we are indebted for a careful histological
investigation of these forms. It will be noted that in addition to
photogenic and lens tissues there are various types of reflector cells
and a line of pigment about the whole inner surface of the organ to
effectively screen the animal's tissues from the light. In one form
(Fig. 30) chromatophores are found about the region where the light is
emitted and these no doubt serve as color filters. There are also an
abundant blood supply and nerves passing to the organ. Figs. 30 and 31
are sections through light organs of fishes.
We thus see that light organs may be very simple and also very
complicated. The latter must have evolved from the former, although it
is not always possible to point out the intermediate stages. It is not
within the scope of this book to discuss bioluminescence in its
evolutionary aspects. It may be worth while, however, to point out
briefly what is known concerning the use of the light to the animal.
There are four possibilities.
[Illustration: FIG. 29.--Sectional view of photogenic organ of a squid,
_Abraliopsis_ (_after Chun_.) _refl^1_, _refl^2_, reflectors; _lac._,
lacunar spaces; _chr._, pigment screen of chromatophores; _chr.^1_,
chromatophore; _phot._, photogenic cells; _l_, lens; _co._, cuticle;
_v_, blood vessel; _fibr._, connective tissue.]
(1) The light may be of no use whatever, purely fortuitous, an
accompaniment of some necessary or even unnecessary chemical reaction.
This appears to be the case in the luminous bacteria and fungi and
perhaps the great majority of forms which make up the marine plankton,
_Noctiluca_, dinoflagellates, jelly-fish, ctenophores and even the
sessile sea pens.
[Illustration: FIG. 30.--Sectional view of photogenic organ of a squid,
_Calliteuthis_ (_after Chun_). _phot._, photogenic cells; _l_, _l^1_,
lens; _n_, nerve; _spec._, "Spiegel"; _pg._, pigmented screen; _c.
fusif._, spindle-shaped reflector cells; _chr._, chromatophore color
screen.]
[Illustration: FIG. 31.--Sectional view of photogenic organ of a fish,
_Stomias_ (_after Brauer_). _p_, pigment screen; _dr_, _dr^1_,
photogenic gland cells; _l_, lens.]
We know that luminous bacteria occasionally lose the power of lighting
and that on certain culture media they develop as non-luminous forms.
Luminescence is not indispensable to them. The same is true of some of
the fungi but _Noctiluca_ and other animals are not known in a
non-luminous condition, although we can see no definite value to the
organism of this power of luminescence.
[Illustration: FIG. 32.--Sectional view of photogenic organ of a fish,
_Argyrophelecus affinis_ (_after Brauer_). _p_, pigmented screen; _dr._,
photogenic cells; _r_, _r^1_, reflector?; _l_, lens?; _s_, sclera; _g_,
connective tissue.]
In the case of sea pens, however, we might suppose that the light acts
as an attraction to small organisms on which the sea pen feeds, although
these creatures only luminesce when stimulated in some way, which rather
detracts from the above suggestion.
(2) The light may act as a warning to scare away predacious animals
which would otherwise feed on the luminous organism. Perhaps this is the
case in the sea pens, although these forms possess nematocysts which
should serve as adequate protection. The marine worm, _Chaetopterus_, is
brightly luminous and lives its whole life in an opaque parchment tube.
If this tube were torn open by a predacious form we might conceive that
the attacking animal would be alarmed by the light and refrain from
destroying the worm. The _Chaetopterus_, however, could not rebuild
another tube and its light would only protect it in the night time.
These cases will suffice to indicate the difficulties and perplexities
of the problem. Perhaps we may add one more guess and suppose that the
light of certain fishes is actually for blinding or distracting their
enemies or blinding the forms on which they feed. Until this use of
luminous organs has actually been observed, we can give little credence
to it.
(3) The light may serve as a means of recognition or a sex signal to
bring the sexes together for mating. It would seem from the work of Mast
and of McDermott that this is the case in the common fireflies and it
may be the case in the toad-fish, _Poricthys_, which is only luminous in
the spawning season and in the worm, _Odontosyllis_, of Bermuda, which
is brilliantly luminous while swarming when the eggs and sperm are shed.
It is non-luminous at other times (Galloway and Welch, 1911.)
(4) Finally, it is possible that animals with complex luminous organs,
such as squid, fish and shrimp, actually use these as lanterns. It is
significant that most of them are deep sea forms, living in a region of
perpetual darkness, and it is perfectly logical to suppose that they
make use of their light organs for illuminating purposes.
The whole problem of the use and purpose of luminous organs is an
exceedingly complex and difficult one. We have, perhaps, said enough to
indicate this and may add that in most cases, so far as opinion is based
on actual evidence and observation, that of the layman is of as great
value as that of the scientist.
CHAPTER V
THE CHEMISTRY OF LIGHT PRODUCTION, PART I
Two experiments, both performed very early in the history of
Bioluminescence, are of great importance in understanding the nature of
animal light. Boyle (1667), as already mentioned, proved the necessity
of air for the luminescence of wood and fish and Spallanzani (1794)
showed that parts of luminous medusae gave no light when dried but if
moistened again would emit light as before. We see then, that air
(oxygen), water, and some photogenic substance are necessary for the
light production. Spallanzani's experiment, which has been confirmed for
a great many luminous forms, shows also that animal luminescence is not
a _vital_ process, in the same sense that the conduction of a nerve
impulse is a vital process. A nerve loses its characteristic property of
conduction on drying or maceration while luminous cells still possess
the power to luminesce after drying or maceration. Using the terminology
of the older physiology we may say that "living protoplasm" is not
necessary for light production.
The experiments of Boyle (1626-91) are of great interest, especially
those in which he studied the behavior of shining wood under the
receiver of his air pump. On October 29, 1667, he wrote:
"Exp. I.: Having procured a Piece of _shining Wood_, about the bigness
of a groat or less, that gave a vivid Light, (for _rotten Wood_) we put
it into a middle sized _Receiver_, so as it was kept from touching the
Cement; and the _Pump_ being set a-work, we observed not, during the 5
or 6 first Exsuctions of the Air, that the splendor of the included Wood
was manifestly lessened (though it was never at all increased;) but
about the 7th Suck, it seemed to glow a little more dim, and afterwards
answered our Expectation, by losing of its Light more and more, as the
Air was still farther pumped out; till at length about the 10th
Exsuction, (though by the removal of the Candles out of the Room, and by
black Cloaths and Hats we made the place as dark as we could, yet) we
could not perceive any light at all to proceed from the _Wood_.
"Exp. II.: Wherefore we let in the outward Air by Degrees and had the
pleasure to see the seemingly extinguished Light revive so fast and
perfectly, that it looked to us almost like a little Flash of Lightning,
and the Splendor of the Wood seemed rather greater than at all less,
than before it was put into the Receiver."
Boyle proved that light from the wood was able to pass a vacuum and
later showed that "shining fish" behaved as the "shining wood," but that
a piece of white hot iron would not regain its light on readmitting air
to the exhausted receiver and that the iron lost its glow under the
air-pump merely because it cooled off. A piece of glowing coal, however,
did lose its light in the absence of air and regained it on again
admitting air, provided the air had not been removed for too long. Boyle
was apparently impressed with the similarity of the light giving process
in glowing coal and shining wood as he draws a comparison between the
two which brings out the fundamental similarity of combustion
processes.
"Resemblances:
VII. The Things wherein I observed a Piece of _shining Wood_ and a
_burning Coal_ to agree or _resemble_ each other are principally these
_five_:
1. Both of them are _Luminaries_, that is, give _Light_, as having
it (if I may so speak) _residing in them_; and not like
_Looking-glasses_, or _white Bodies_, which are conspicuous only by
the _incident Beams_ of the _Sun_, or some other _luminous Body_,
which they _reflect_....
2. Both _shining Wood_ and a _burning Coal_ need the Presence of the
Air (and that too of such a _Density_ to make them continue
_shining_)....
3. Both _shining Wood_ and a _burning Coal_, having been deprived,
for a Time, of their _Light_, by the withdrawing of the contiguous
_Air_, may presently recover it by letting in fresh _Air_ upon
them....
4. Both a _quick Coal_ and _shining Wood_ will be easily quenched by
_Water_ and _many other Liquors_....
5. As a _quick Coal_ is not to be _extinguished_ by the Coldness of
the _Air_, when it is greater than ordinary; so neither is a Piece
of _shining Wood_ to be deprived of its _Light_ by the same Quality
of the _Air_....
Differences:
1. The first Difference I observed betwixt a _live Coal_ and a
_shining Wood_ is, that whereas the _Light_ of the _former_ is
readily _extinguishable_ by _Compression_ (as is obvious in the
Practice of suddenly _extinguishing_ a piece of _Coal_ by treading
upon it), I could not find that such a _Compression_ as I could
conveniently give without losing sight of its operation, would _put
out_, or much injure the _Light_, even of small Fragments of
_shining_ Wood....
2. The next _Unlikeness_ to be taken notice of betwixt _rotten Wood_
and a _kindled Coal_ is, that the latter will, in a very few
_Minutes_, be totally _extinguished_ by the withdrawing of the
_Air_; whereas a Piece of _shining Wood_, being eclipsed by the
Absence of the _Air_, and kept so for a Time, will immediately
_recover_ its _Light_ if the _Air_ be let in upon it again within
half an hour after it was first withdrawn....
3. The next _Difference_ to be mentioned is, that a _live Coal_,
being put into a small close Glass, will not continue to _burn_ for
very many _Minutes_; but a Piece of _shining Wood_ will continue to
shine for _some_ whole _Days_....
4. A _fourth Difference_ may be this: that whereas a _Coal_, as it
_burns_, sends forth Store of _Smoke_ or _Exhalations_, _luminous
Wood_ does not so.
5. A _fifth_, flowing from the former, is, that whereas a _Coal_ in
_shining_ wastes itself at a great Rate, _shining Wood_ does not....
6. The last Difference I shall take notice of betwixt the bodies
hitherto compared is, that a _quick Coal_ is actually and vehemently
hot; whereas I have not observed _shining Wood_ to be so much as
sensibly _lukewarm_."
It should be clearly borne in mind that if we place luminous organisms,
say bacteria or fungi, in an atmosphere devoid of oxygen and find that
no light is produced, this may merely mean that certain functions of the
cell are interfered with, including light production, but does not
necessarily indicate that oxygen is actually used up in the photogenic
process. If we find, however, that extracts of luminous cells or
luminous secretions devoid of cells cease to light when the oxygen is
removed and again luminesce when it is returned, we may be quite certain
that the photogenic process itself requires free oxygen. As luminous
extracts of fireflies, pennatulids, ostracods, _Pholas_ and others give
off no light when the oxygen is removed, we may safely conclude that for
these luminescences, oxygen is necessary. Bacteria, fungi, and
_Noctiluca_, whose light also disappears in absence of oxygen, although
they are whole cells, we may by analogy also assume to require oxygen in
the photogenic process.
Some of the earlier workers on fireflies and _Noctiluca_ obtained light
even after placing these organisms in absence of oxygen, but they did
not realize how low is the amount of oxygen necessary to produce light.
It is difficult to remove traces of oxygen from the water, traces which
are nevertheless sufficient to cause luminescence. If the organisms are
numerous, as in an emulsion of luminous bacteria, they will themselves
use up all the oxygen and the liquid soon ceases to glow except at the
surface in contact with air. We may gain an idea of the amount of oxygen
necessary for luminescence from an experiment of Beijerinck (1902). He
mixed luminous bacteria with an emulsion of clover leaves containing
chloroplasts and kept the two in the dark until all the oxygen was used
up and the bacteria ceased to glow. If now a match was struck for a
fraction of a second, sufficient oxygen was formed by photosynthesis to
cause the bacteria to luminesce for a short time.
Exact figures on the minimal concentration of oxygen for luminescence
cannot be given. The luminescent secretion of _Cypridina hilgendorfii_
will still give off much light if hydrogen containing only 0.4 per cent.
of oxygen is bubbled through it, _i.e._, a partial oxygen pressure of
1/250 atmosphere (3.04 mm.Hg). However, addition of a fresh emulsion of
yeast cells to a glowing _Cypridina_ secretion is sufficient to rapidly
extinguish the light, because the yeast is capable of utilizing the last
trace of oxygen in the mixture. Light only appears when, by agitation,
we cause more air to dissolve. The minimal concentration of oxygen for
luminescence of _Cypridina_ lies somewhere between 3.04 mm. and the
amount which living yeast fails to extract from solution, a
concentration approaching zero. It is probably nearer the latter figure.
As the oxygen pressure is increased from 0 to about 7 mm., the intensity
of the _Cypridina_ luminescence increases and at the latter figure the
light is just as bright as if the solution were saturated with air (152
mm.O_{2}). Thus, the luminescence requires only a low pressure of oxygen
and the similarity to the saturation of haemoglobin with oxygen is
obvious. Just as haemoglobin is nearly saturated with oxygen at low
pressures and becomes bright red in color, so the luminous material
becomes saturated with oxygen at low pressures and glows intensely.
Boyle also made many experiments to show that air was necessary for the
life of animals and the germination of seeds and showed that repeatedly
respired air was unfit for further breathing. About the same time R.
Hooke discovered the true meaning of respiratory movements and by
forcing a blast of air continuously through the lungs with bellows, was
able to keep animals alive. He concludes "that as the bare Motion of the
_Lungs_, without fresh air, contributes nothing to the life of the
Animal, he being found to survive as well as when they were not moved as
when they were; so it was not the Subsiding or Movelessness of the
_Lungs_ that was the immediate cause of death, or the stopping of the
circulation of the Blood through the _Lungs_, but the Want of a
sufficient Supply of fresh Air." The cause of death on collapse of the
lungs could not be better stated to-day. Thus combustion, respiration
and luminescence of flesh or wood were early recognized as related
phenomena.
Although the "gas sylvestre" (CO_{2}) of burning charcoal and
fermentation of wine was known to van Helmont (1577-1644) and Mayow
(1646-1679) in 1674 showed that "spiritus nitroaerens" (oxygen) was
responsible for the life of animals and for combustion, a century
elapsed before the true significance of these gases became known. In the
meantime the phlogiston theory of combustion had been developed, Black
(1728-1799) in 1755 had rediscovered carbon dioxide ("fixed air") in the
expired air and Priestley (1733-1804) and Scheele (1742-1786) had both
rediscovered oxygen ("dephlogisticated air") in 1774. About the same
time Lavoisier overthrew the phlogiston doctrine and showed that in the
combustion of organic substances water and CO_{2} are formed.
Later it was realized that this slow combustion did not take place in
the lungs, or in the blood, but in the tissues cells themselves and
respiration in the chemical sense has come to mean this universal slow
combustion in the cells of the body rather than the breathing movements
of the lungs themselves. In anaerobic respiration, CO_{2} is given off,
but no oxygen absorbed. In aerobic respiration, oxygen is absorbed and
CO_{2} given off. In addition we know of many substances which oxidize
by taking up oxygen without giving off CO_{2}. We have seen that oxygen
must be absorbed for luminescence of animals and we may now inquire
whether CO_{2} is given off and the relation between respiration and
light production.
To determine if CO_{2} is given off during luminescence it is necessary
to work with fairly pure luminous materials, obtained from luminous
organisms. It is impossible to use the living organisms themselves as
the CO_{2} continually respired becomes a very disturbing factor. From
_Cypridina_, a small crustacean, two materials soluble in water may be
prepared (_luciferin_ and _luciferase_), which will give a brilliant
luminescence on mixing. It is possible to determine the H-ion
concentration of the two solutions separately and of the mixture of the
two after the luminescence has occurred.
If CO_{2} is produced during luminescence the H-ion concentration of the
luminous solution should increase. Measurements made electrometrically
with the hydrogen electrode have failed to demonstrate any increase in
acidity. The PH of both solutions and of a mixture of the two is 9.04.
This would indicate that CO_{2} is not produced. As both luminous
solutions contain proteins and the luminous substances themselves are
probably proteins, which have a high buffer value, a method of this kind
is none too sensitive. However, we can definitely state that not enough
CO_{2} is produced to be detected and that this may be due to the buffer
action of the luminous substances themselves. After all, unless
luminescence is connected with respiration, we should hardly expect
CO_{2} to be produced.
Another method of testing CO_{2} production is to measure the amount of
heat produced during luminescence. Substances burned during respiration
give off considerable heat, one gram of glucose to CO_{2} and H_{2}O, as
much as 4000 calories. We have seen in Chapter III that no infra-red
radiation is produced in the light of the firefly. This does not mean,
however, that no heat is produced by the reaction which produces the
luminescence. A temperature change of a few thousandths or hundredths of
a degree would evolve no measurable radiation. Coblentz (1912) first
studied the problem of heat production in the firefly, using a
thermocouple as the measuring instrument. He came to the conclusion that
the temperature of the insect was slightly lower than the temperature of
the air and that the luminous segments were slightly hotter than the
non-luminous segments, whereas a dead firefly is of the same temperature
as its surroundings. No definite increase or decrease in temperature
could be established during the flash of the firefly. However, further
work on the firefly is much to be desired.
The use of a living animal for such measurements introduces a possible
source of error in that any contraction of the muscles of the animal
will produce heat which may add to an increase or mask a decrease of
temperature during luminescence. Utilization of extracts of luminous
animals containing the luciferin and luciferase mentioned above avoids
the complications due to muscular contraction. By bringing the solutions
of luciferin and luciferase to the same temperature and then mixing
them one can measure any increase or decrease of temperature which
occurs during the luminescence which results from mixing. We can thus
gain some idea of the heat of oxidation of luciferin.
As a determination of heat production is of considerable interest the
method will be given in some detail. Although the experiment sounds very
simple, it is actually somewhat difficult to carry out. The attainment
of temperature equilibrium between two solutions is very slow when one
wishes to obtain them to within 0.001 deg. C. of the same temperature. After
many attempts, the following arrangement of apparatus (Fig. 33) was
found most satisfactory. About 10 c.c. luciferin solution was placed in
the inner tube (_D_) of a special non-silvered thermos bottle (_A_).
About 1 c.c. of luciferase solution was placed in a very thin-walled
glass tube (_E_) which was immersed in the luciferin solution and
connected with a small motor so that it could be slowly but constantly
rotated, thus stirring the solutions. Thermocouples (_L_ and _M_) of
advance (.008 in)--copper (No. 30, _B_ and _S_, enamel insulated) wire
were paraffined and placed in each tube and the copper wires connected
through a copper double throw switch (_C_) with a Leeds and Northrup
d'Arsonval wall galvanometer (No. 34637, silver strip suspension) of 35
ohms resistance and 310 megohms sensitivity. The constant temperature
junctions (_N_) were placed in a large Dewar flask (_B_) filled with
water at approximately the same temperature as the luciferin solution.
One mm. galvanometer scale division represented 0.003 deg. C. and the
division readings could be estimated to tenths. By means of a glass rod
(_F_) placed in the tube containing luciferase solution, this tube
could be broken and the luciferase and luciferin solution mixed.
[Illustration: FIG. 33.--Apparatus for determining heat production
during luminescence of luciferin. A, special thermos tube. B, Dewar
flask for constant temperature junctions. C, double throw switch. D,
tube containing luciferin solution. E, tube containing luciferase
solution. F, glass rod for breaking E. G, rubber stopper with groove, K,
for pulley cord. H, cork closing thermos tube. J, brass sleeve in H
allowing rotation of E. L, thermojunction in luciferase solution. M,
thermojunction in luciferin solution. N, constant temperature
junctions.]
[Illustration: FIG. 34.--Curve showing temperature change when two tubes
containing water at the same temperature are mixed. 0.1 galvanometer
scale division = 0.003 deg. C. Dots represent readings of thermocouple in
tube D; crosses readings of thermocouple in tube E.]
It was found that even after the luciferase and luciferin solutions
came to the same temperature within the thermos bottle, this was not
necessarily the same as that of the room and a slow rise or fall
occurred as indicated by a slow drift of the galvanometer coil. Readings
of each thermocouple on the galvanometer scale were therefore taken at
one-minute intervals for some time before and after mixing the luciferin
and luciferase solutions and plotted as curves. Control experiments were
also carried out in exactly the same manner as the luciferin-luciferase
experiments, but water was placed in the two tubes instead of luciferin
and luciferase. Figs. 34 and 35 give typical experiments with water and
with luminescent solutions, respectively.
[Illustration: FIG. 35.--Curve showing temperature change when luciferin
and luciferase solutions at the same temperature are mixed. 0.1
galvanometer scale division = 0.003 deg. C. Dots represent readings of
thermocouple in luciferin solution; crosses, readings of thermocouple in
luciferase solution.]
With both control (water) and luciferin experiments there was a slight
rise in temperature on mixing the liquids in the two tubes. The average
rise of five control (water) experiments was .0054 deg. C. and the average
rise of five luciferin experiments was .0048 deg. C.
The average rise in temperature is no doubt due to heat from friction in
mixing of the liquids and breaking of the glass tube. The difference in
the average rise of control and of luciferin experiments is so small
(.0006 deg. C.) as to have little significance. We may therefore conclude
that if any temperature change occurs during the luminescent reaction it
is certainly less than 0.001 deg. C. and probably less than 0.0005 deg. C., too
small to be measured by this method.
To prepare the luciferin solution, two grams of dried _Cypridina_ were
dissolved in 20 c.c. hot water and 10 c.c. of this 10 per cent. solution
was used in the thermos bottle in the above experiments. If we assume
that 1 per cent. of the dried _Cypridina_ is luciferin, 0.01 gram of
luciferin on oxidation was not able to raise the temperature of the 10
c.c. (in reality 11 c.c., since 1 c.c. luciferase solution was mixed
with the 10 c.c. luciferin solution) .001 deg. C. This means that 1 gram
luciferin liberates _at least less_ than 10 calories during the
luminescence accompanying oxidation.
Since 1 gram glucose liberates 4000 calories on complete oxidation to
CO_{2} and H_{2}O, it will be seen that the oxidation of luciferin is a
very different type of reaction from the oxidation of glucose. As we
shall see, it is probably similar to the oxidation of reduced haemoglobin
or the oxidation of leuco methylene-blue to methylene blue. According to
Barcroft and Hill (1910), 1.85 calories are produced per gram of
haemoglobin oxidized. I have been unable to find figures for the heat
exchange during oxidation of leuco-dyes, but it is no doubt also small.
Since luciferin evolves no measurable amount of heat on oxidation, we
have very good evidence in support of that obtained by electrometric
measurements of H-ion concentration, that no carbon dioxide is produced
during luminescence of luminous animals.
In most animal cells it is perfectly clear that luminescence does not
accompany respiration, since respiration is a continuous process,
whereas light is only produced on stimulation. It is true that on
stimulation respiration is accelerated, and we might suppose that
luminescence is an accompaniment of accelerated respiratory oxidations;
but this is not the case, for in luminous animals a rise in temperature
of ten degrees centigrade will accelerate the respiratory oxidations 250
per cent. without necessarily causing the production of light.
In fungi and bacteria, on the other hand, which continually emit light,
it is quite natural to suppose that the light is an accompaniment of
respiration, just as we know the heat of these forms to be. This view
was accepted by such of the earlier workers as Fabre in 1855, who found
that luminous portions of a mushroom, _Agaricus olearius_, gave off more
CO_{2} (4.41 c.c. CO_{2} per gram in 36 hours at 12 deg. C.) than
non-luminous portions (2.88 c.c. CO_{2} per gram in 36 hours at 12 deg. C.).
This experiment has never been repeated and there are many reasons
besides luminescence why one piece of fungus might have a more rapid
respiratory rate than another piece. It is not true that rapidly
respiring plant tissues, such as germinating seeds or the spadix of
_Araceae_, are luminous, although they produce considerable heat.
On the other hand, it is very easy to prove that luminescence, even in
bacteria, is not connected with respiration. Thus, Beijerinck (1889 _c_)
found that of several species of luminous bacteria studied by him, one,
_Bacterium phosphorescens_, was a facultative anaerobe and would grow,
_i.e._, multiply, but not luminesce in the absence of oxygen. Some
forms, ordinarily producing light, will grow, but fail to luminesce at
high temperatures. Beijerinck (1915) has recently found that these
individuals may, by continued cultivation at high temperatures, form
non-luminous strains which fail to luminesce when again brought into
lower temperatures, favorable for luminescence. These non-luminous
mutants occasionally give rise to atavistic brilliantly luminous forms.
Beijerinck also finds that after exposure of _Photobacter splendidum_ to
ultra-violet or strong sunlight, radium or mesothorium rays,
luminescence continues but no growth occurs. There is thus ample
evidence that growth and respiration are properties quite distinct and
separable from luminescence. Indeed, respiration increases continuously
up to a relatively high maximum whereas luminescence falls off rapidly
above a relatively low optimum. McKenney (1902) found also that _Bacillus
phosphorescens_ could grow rapidly in 0.5 per cent. ether without
producing light.
Luminescence has been compared in bacteria to pigment formation, as
rather definite cultural conditions are necessary for realization of
both chromogenic and photogenic function. Some pigment-formers, as
_Bacillus pyocyaneus_, which produces a water-soluble green pigment,
remain colorless under anaerobic conditions. A colorless chromogen is
formed, which oxidizes to the green pigment in the air. If this
colorless chromogen produced light during its oxidation as well as green
pigment, we would have a case of both chromogenic and photogenic
function combined in one species of bacterium. Luminescence involves
something more than respiration, an oxidation of a very definite and
particular kind.
Since Spallanzani's observation that the luminous material of medusae
could be dried, and upon moistening would again give light, many
confirmatory observations have been made on other forms. _Pyrosoma_,
_Pholas_, _Phyllirrhoe_, fireflies, _Pyrophorus_, copepods, ostracods,
pennatulids, fungi, and bacteria can all be dessicated and the
photogenic material preserved for a greater or less time. In a
dessicator filled with CaCl_{2}, dried luminous bacteria lose, after a
few months, their power to give light on being moistened. On the other
hand, ostracods and copepods will still luminesce after years of
dessication. The luminous material in the latter case appears capable of
indefinite preservation, but it is possible that the quick loss of
photogenic power with dried luminous bacteria is merely an indication
that they contain very little photogenic substance and that the dried
ostracods would also in time lose their power to luminesce. It is
certainly a fact that the amount of luminous material in a single gland
cell of an ostracod is vastly greater than that in the same mass of
bacterial colony.