Файл: Pye D. Polarised light in science and nature (IOP)(133s).pdf

Figure 9.8. The outer, light-sensitive regions of retinal cone cells in the anchovy. The edges of the sensitive membranes are indicated by the close parallel lines (though there are very many more than shown here). The double-headed arrows show the planes of the membranes, which are tilted to lie along the normal light path and are therefore likely to be dichroic. There are two kinds of cell that alternate along the vertical rows of cones within the retina. The short forked cones have vertical membranes and should respond to vertically polarised light, while the long pointed cones have horizontal membranes and should respond to horizontally polarised light. This orthogonal arrangement is unknown among other vertebrates and probably forms a polarisation analyser. Light that has entered the eye through the lens comes from the left as shown by the larger arrows.

to the twisted arrangement of the cell bundles in insect eyes. So far this arrangement appears to be unique among vertebrates and the reason for it is unknown.

Another possibility involves the eyes of some fish such as trout, in which the retina retains a fold called the embryonic fissure into later life. In some cases this fold holds small numbers of cone cells that are therefore sideways on to the light path and would be expected to be dichroic. But no correlation has been attempted between this arrangement and the ability to detect polarisation, nor would it enable the fish to analyse more than a tiny part of the visual field.

Nevertheless evidence has steadily accumulated to suggest that many other non-mammalian vertebrates may be sensitive to polarisation and even observe its direction. For instance a variety of fish including trout and goldfish show spontaneous responses such as orientation to the direction of polarisation. Salamanders and pigeons can be trained

migratory birds appear to use the sky compass (chapter 6) after dusk, and during development may ‘set’ an internal sense of the earth’s magnetic field by comparison with the polarised sky compass.

The mechanism is still not clear but it seems that polarisation dependence in these vertebrates may be associated with ‘double cones’ in the retina. These comprise two similar cone cells in an intimate association, the combined pair having an elliptical cross section. It has been suggested that these pairs might somehow conduct light in a birefringent way and therefore may be polarisation dependent. This suggestion is supported by the layout of the double cones in the retina, where they are set in a square mosaic of tetrads with the long axes of the ellipses set in two orthogonal directions. Furthermore the polarisation sensitivity of the Green Sunfish, Lepomis cyanellus, has been found to be best at the same red wavelength as light absorption by the double cones, whereas in this fish no other cones have the same pigment. It also seems to be significant that double cones are found in fishes, amphibia, reptiles and birds but not in mammals, where no case of polarisation sensitivity has (yet) been discovered.

The story is far from clear, however. Electrical recordings from single cells and nerve fibres in trout and goldfish have shown that green and red double cones respond to the same direction of polarisation although they are orientated at right angles to each other. A common basis of birefringence due to asymmetry of cross section seems unlikely therefore. Also the ultraviolet cones (but not the blue ones) respond strongly to polarisation direction although they are not formed into double cone pairs.

Finally, there is one way in which humans are sometimes said to be able to detect strong polarisation directly, without reference to reflections or scattering. The effect is not strong and I myself, in common with others, have great difficulty seeing it. By gazing at an even field of strongly polarised light, especially if it is blue in colour (such as a clear sky at right angles to the sun) one is supposed to see a small faint figure, called Haidinger’s brush after its discoverer who described it in 1844. It consists (figure 9.9) of two yellow, brushlike patterns back to back and with blue areas between. The whole figure is about 3 ◦ wide (the full moon is about half a degree wide) and the yellow–brown wings are aligned at right angles to the direction of polarisation. I have only seen it very faintly after staring for some time with one eye through a

Figure 9.9. An impression of the pattern known as Haidinger’s brushes that is sometimes seen when people look at a patch of polarised light (from various sources). The barlike figure (dashes) with open fuzzy ends, the ‘brushes’, is faintly yellow–brown while the (dotted) areas either side of its waist are bluish. The axis of the figure is at right angles to the direction of polarisation, shown by the arrow, and extends for about 3◦.

polaroid film at a brightly lit sheet of white paper. Even then it only works for me when the polaroid is suddenly rotated by 90 ◦ or so and the figure appears as it rotates too. It is also said that circularly polarised light can produce brushes, aligned lower left to upper right for right-hand circular polarisation and upper left to lower right for left-hand circular polarisation. I cannot vouch for this.

There is no consensus on the explanation for these figures although they are commonly attributed rather vaguely to ‘dichroism in the fovea’, or that part of the retina responsible for the most detailed central vision and sometimes called the yellow spot. The usefulness of the figures is probably negligible and they are mentioned here simply because some people say that humans can see the effects of polarisation after all. To see a small figure, however, is not at all the same as being able to see the property of polarisation in an image. What the world might actually look like to animals which have that ability is, after all, something we can only imagine. Our knowledge of the various phenomena described in this book is only the beginning. We can only dream about its part in any whole sensory experience.

Some recommendations for further reading

Bunn C 1964 Crystals: Their Role in Nature and in Science (New York: Academic)

Greenler R 1980 Rainbows, Halos and Glories (Cambridge: Cambridge University Press) (paperback edn 1989).

Gribble C D and Hall A J 1985 A Practical Introduction to Optical Mineralogy

(London: Allen and Unwin) (paperback)

Hartshorne N H and Stuart A 1960 Crystals and the Polarising Microscope

3rd edn (London: Arnold)

Konnen G P 1985 Polarised Light in Nature (Cambridge: Cambridge University Press) (first published 1980 in Dutch by Thieme & Cie-Zutphen, The Netherlands)

Land E H 1951 Some aspects of the development of sheet polarizers J. Opt. Soc. Am. 41 957–63

Lowry T M 1964 Optical Rotatory Power (New York: Dover) (1st edn 1935 (London: Longmans Green))

Lynch D K and Livingston W 1995 Color and Light in Nature (Cambridge: Cambridge University Press)

Lythgoe J N 1979 The Ecology of Vision (Oxford: Clarendon)

Minnaert M 1940 The Nature of Light and Colour in the Open Air (London: Bell) (paperback edn 1954 (New York: Dover))

Robinson P C and Bradbury S 1992 Qualitative Polarized-Light Microscopy (Royal Microscopical Society Microscopy Handbook 9) (Oxford: Oxford University Press)

Shurcliff W A and Ballard S S 1964 Polarized Light (Princeton, NJ: Van Nostrand) (paperback)

Smith H G 1956 Minerals and the Microscope 4th edn, revised by M K Wells (London: Murby) (paperback)

Spottiswoode W 1874 Polarisation of Light (London: Macmillan)

119

120 Some recommendations for further reading

van de Hulst H C 1957 Light Scattering by Small Particles (New York: Wiley) (paperback edition 1981 (New York: Dover))

von Frisch K 1950 Bees: Their Vision, Chemical Senses and Language (Ithaca, NY: Cornell University Press) (paperback and later editions in UK)

——1954 The Dancing Bees (London: Methuen) (University Paperback edition 1970)

Waterman T H 1981 Polarization sensitivity in Handbook of Sensory Physiology vol VII/6B, Comparative Physiology and Evolution of Vision in Invertebrates, B: Invertebrate Visual Centers and Behavior I ed H Autrum (Berlin: Springer) pp 281–469

Wehner R 1987 ‘Matched filters’—neural models of the external world

J. Comparative Physiol. A 161 511–31

Wood E A 1964 Crystals and Light: An Introduction to Optical Crystallography

(Princeton, NJ: Van Nostrand) (revised paperback edn 1977 (New York: Dover))

Polaroid materials of various kinds are available from many optical and general scientific suppliers. I am especially grateful for helpful advice, information and small quantities of unusual products that have been supplied by Optical Filters Ltd, The Business Centre, 14 Bertie Road, Thame, Oxon OX9 3FR, UK (01844-260377).