Fluorescence

“ Our data are, however, consistent with the alternative hypothesis, which states that fluorescence can serve a visual contrast function when the wavelengths emitted by fluorescence are rare or absent from the ambient light. Visual contrast enhancement offers an intriguing new adaptive function for fluorescent pigments in marine environments, which calls for investigations of the physical properties, perceptive abilities and behavioural consequences of signalling using locally rare colour hues. ”

“ This feature is unique to fluorescence and other forms of luminescence (e.g. chemi- or bioluminescence [16] ) . Virtually all animal colours, however, merely reflect or transmit light that is not absorbed. Consequently, they display a down-sampled subset of the ambient spectrum, which is why they are called subtraction colours. This applies to pigments as well as structural colours [17] , [18] . The key question is under what conditions additive fluorescent coloration can be significant for colour vision, given the evolutionary success of subtraction colours. ”

“ Fluorescent light is only perceived so long as the fluorescent substance is illuminated by the exciting rays. As soon as the light falling on it is obstructed, the colored shimmer vanishes. It is only in the case of some fluorescing solid substances, as, for example, fluorspar and uranium glass, that, with the aid of appropriate apparatus (Becquerel's phosphoriscope) , a very short continuance of the fluorescence may be observed to take place in the dark. ”

“ However, various ‘super-resolution’ methods have allowed researchers to overcome this diffraction limit for fluorescence imaging, which is the most popular form of microscopy used in the life sciences. This approach involves tagging the biomolecules of interest with fluorescent molecules, such as green fluorescent protein (GFP) , so that they can be identified in cells. An excitation laser then drives the fluorescent molecule, which is also known as a fluorophore, into an excited state: after a short time, the fluorophore can return to its ground state by releasing a fluorescence photon. ”

“ Fluorescence, or the internal dispersion of light, though far from possessing the beauty or extensive consequences of polarized light, is scarcely less wonderful. A variety of substances, such as canary-glass, a solution of sulphate of quinine, fluor-spar, and 196a great number of organic substances, have the property of diminishing the refrangibility of light by internal dispersion, consequently of increasing the length of the waves, and lowering the colour in the prismatic scale ”

“ We suspect that the observed difference between the two depths involves differences in the number of melanophores covering the iris, the number of fluorescent chromatophores and/or the concentration of fluorescent pigment within the fluorescent chromatophores. Because we did not correct for the size of the fluorescent patch, we cannot exclude that fluorescent patch size may also have contributed to the observed effect. The differences in fluorescence brightness could originate from phenotypic plasticity during development or in the adult stage, or due to local genetic adaptation. ”

“ The eight study species under fluorescence and white light conditions. The first two columns were taken in the laboratory under standard fluorescence photography conditions, showing the individuals with the minimum (left column) and maximum (right column) fluorescence brightness among all sampled individuals. The third column shows individuals under standard white light conditions in the laboratory. Pictures were all taken under the same light conditions, explaining slight overexposure effects in species with strongly fluorescent eyes (triplefins H. striata and T. delaisi) . ”

“ Just as the loadstone disclosed magnetism and amber frictional electricity, so did Iceland spar show the existence of double refraction, and sulphate of quinine the phenomenon of fluorescence. When one such startling instance has drawn the attention of the scientific world, numerous less remarkable cases of the phenomenon will be detected, and it will probably prove that the property in question is actually universal to all matter. ”

“ Such is the singular phenomenon of internal dispersion, degraded light, or fluorescence. The brightness is by no means due to phosphorescence, because the canary-glass only shines when under the influence of the active or blue rays, whereas phosphorescent bodies shine by their own light—the latter has independent, the former dependent, emission ”

“ Fluorescence is characterised by wavelength of maximum emission and fluorescent brightness in Fig. 2.
Wavelength of maximum fluorescent emission across species. Size of circles depicts relative fluorescent brightness Microspectrophotometry
Microspectrophotometric data for all photoreceptor classes found in the fairy wrasse Cirrhilabrus solorensis are summarised in Table 1 and displayed in Figs. 3 and 4. All visual pigment absorbance spectra are considered to represent vitamin A1-based visual pigments (i.e. rhodopsins) , according to the fit of the data to A1 visual pigment templates [31] . ”

“ The role of fluorescence in colour vision (a)
Natural luminescence, whether fluorescence or chemiluminescence, has one drawback: it is weak compared with the ambient sunlight in the same spectral range. As a consequence, the bioluminescent eyes of flashlight fish, for example, are only functional in twilight or darkness [19] . Such restrictions also apply to animal fluorescence. Although short-wavelength light is required to induce it, there should be little if any ambient light at the longer wavelengths where the fluorescent light is emitted. ”

“ Professor Ångström, in his Aurora Memoir (discussed elsewhere) , in discussing the yellow-green line, considers the only probable explanation to be that it owes its origin to fluorescence or phosphorescence. He says that some fluorescence is produced by the ultra-violet rays; and adds, “an electric discharge may easily be imagined, which, though in itself of feeble light, may be rich in ultra-violet light, and therefore in a condition to cause a sufficiently strong fluorescent light.” And he refers to the fact that oxygen and some of its compounds are phosphorescent. ”

“ Bence Jones and Dupré, it would seem that there is some substance in the human body resembling the sulphate of quinine, which causes all the tissues of the body to be more or less fluorescent. All animal infusions show this fluorescence. The crystalline lens of the eye exhibits the effect in a very striking manner. When, for example, I plunge my eye into this violet beam, I am conscious of a whitish-blue shimmer filling the space before me. This is caused by fluorescent light generated in the eye itself. Looked at from without, the crystalline lens at the same time is seen to gleam vividly. ”

“ The rays emitted by the new radio-active bodies cause fluorescence of certain substances. M. Curie and myself first discovered this phenomenon when causing polonium to act upon a layer of barium platinocyanide through aluminium foil. The same experiment succeeds yet more easily with barium containing radium. When the substance is strongly radio-active the fluorescence produced is very beautiful. ”

“ As a consequence, even weak red fluorescence may become visible to an observer with the appropriate sensitivity. This is due to the way in which eyes perceive chromatic contrast; it depends on the ratios of cone photoreceptor types that are stimulated by light coming from an object compared with an adjacent object or background [20] , [28] . Hence, even when quantitatively weak, red fluorescent structures could produce a perceptible colour contrast against the cyan background of the stenospectral zone. ”

“ Scale bar = 500 μm.Spectra of excitation and emission of the fluorescent pigment in fluorescent chromatophores.Fluorescence has a maximum at around 595 nm (dashed line) with an optimal excitation wavelength around 500 nm (solid line) .Distribution of erythrophores, melanophores and fluorescent chromatophores in the interradial membrane of a dorsal fin of E. pellucida.a) Erythrophores (red) and melanophores (black) are visible in bright field microscopy. b) Fluorescent chromatophores appear in fluorescence microscopy. ”

“ The crystals take the shape of cubes, often twinned, and have an easy octahedral cleavage.286 The refraction is single, the index being 1·433. Fluor is noted for its property of appearing of differing colour by reflected and transmitted light, and the phenomenon is in consequence known as fluorescence. The specific gravity is 3·18, and the hardness 4 on Mohs’s scale. Owing to its low refraction and softness, fluor is not suitable for jewellery. Clear colourless material is in demand for particular lenses of microscope objectives. ”

“ Mr. Stokes, who has investigated this class of phenomena, and proposes to call it Fluorescence, from its being naturally seen in fluor-spar, has shown that the peculiar internal dispersion, and the consequent alteration of the colour of the ray, is due to an alteration in its refrangibility. Whether this hypothesis prove to be the correct one or not, it is certain that there exists a set of rays of far higher refrangibility than those seen in the ordinary Newtonian spectrum. ”

“ Lommel distinguishes two kinds of fluorescence. The bodies which exhibit the first kind are those which possess strong absorption bands, of which only one remains appreciable after great dilution. These bodies are always strongly coloured and show anomalous dispersion and (in solids) surface colour. In such cases, the maximum of intensity in the fluorescent spectrum corresponds to the maximum of absorption. Stokes’s law is not obeyed, for a fluorescent spectrum can be produced by means of homogeneous light of lower refrangibility than a great part of the fluorescent light. ”

“ FLUORESCENCE. In a paper read before the Royal Society of Edinburgh in 1833, Sir David Brewster described a remarkable phenomenon he had discovered to which he gave the name of “internal dispersion.” On admitting a beam of sunlight, condensed by a lens, into a solution of chlorophyll, the green colouring matter of leaves (see fig. 1) , he was surprised to find that the path of the rays within the fluid was marked by a bright light of a blood-red colour, strangely contrasting with the beautiful green of the fluid when seen in moderate thickness. ”

“ The first group, named the “luminophore,” is such that when excited by suitable aetherial vibrations emits radiant energy; the other, named the “fluorogen,” acts with the luminophore in some way or other to cause the fluorescence. This theory explains the fluorescence of anthranilic acid (o-aminobenzoic acid) , by regarding the aniline residue as the luminophore, and the carboxyl group as the fluorogen, since, apparently, the introduction of the latter into the non-fluorescent aniline molecule involves the production of a fluorescent substance. ”

“ The heating of the body and the consequent re-emission of heat-waves is not, however, the only process which goes on. In a large number of substances, waves are given out under the stimulus of other waves without any heating of the body at all. In most of these cases the emission stops as soon as the stimulating waves are withdrawn, and in these cases the phenomenon has been called fluorescence. ”

“ The presence of biofluorescence in these deepwater taxa that spend their lives primarily in the dark, beyond the reach of the high-energy blue light necessary for excitation of fluorescence, is curious from a functional perspective. ”

“ All this indicates that the photoprotection hypothesis may be a valid explanation for fluorescence in fish—at least in species where fluorescence is located in sensitive structures. The visual contrast hypothesis, however, offers an attractive alternative for fish in the stenospectral zone. Many marine fishes possess photoreceptors with sensitivities extending into the long-wavelength part of the ambient spectrum, including families with many fluorescent representatives such as wrasses [31] , pipefish [32] and gobies [9] , [33] . ”

“ Fluorescence brightness differed significantly between the euryspectral and stenospectral sampling zones for six out of eight species, indicating that fluorescence is adjusted to depth. In all six species, the fluorescing irides were significantly brighter at greater depth, and none of the species examined showed the opposite pattern. Although we used species from three fish families and seven genera, fluorescent peak emission was very similar at 600–620 nm in all eight species. ”

“ This is, in my opinion, the proof that there is here merely a phenomenon of electric induction producing phosphorescence of the gelatin, and at the same time an electric discharge in the gelatin, and, lastly, the fluorescence of the ambient air, and as in case of the dark discharge of electricity. In my opinion, these are the three agents which determine the decomposition of the silver salts in the sensitive layer. There are no special radiations, no X rays, and no dark light. ”

“ Electric light possesses to a marked degree the property of calling forth fluorescence, as can easily be demonstrated by various experiments. It may hence be assumed that the red lines in the spectrum of northern lights are due to the fluorescence of oxygen in the upper layers of air, caused by powerful electric discharges. ”