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Published online by Cambridge University Press: 14 March 2018
In this paper an attempt is made to state and explain, in a simple manner, the essential features of the phenomena which are seen when plates of pleochroie minerals are examined in ordinary light.
The pleochroie effects exhibited by minerals are usually studied in light of practically normal incidence. For the observation of facial colours, transparent crystals, or, better still, specially orientated cube-shaped pieces of minerals, are examined in ordinary light. In general, by this mode of observation, only facial colours are seen when ordinary light is used, and for the observation of axial colours the incident light must be polarized.
Page 2 note 1 In this paper, the phrase ‘axial colour’ is used in the wide sense as meaning a single-ray colour in a doubly-refracting crystal.
Page 3 note 1 The behaviour of amethystine quartz (p. 12) appears to indicate that, as regards absorption, this is not necessarily the case for all uniaxial crystals.
Page 4 note 1 The best way to do this experiment is to cement the calcite section to one of the plates of phlogopite, as this facilitates manipulation.
Page 6 note 1 Many biaxial minerals, e.g. amazon-stone and black biotite, are practically ‘dichroic’ and are capable of yielding plates which act pleochroically much like basal plates of uniaxial minerals.
Page 7 note 1 ‘Silver-mica’ appears to be generally phlogopite. There are two sorts ; one of which owes its sheen to the presence of rod-like inclusions lying along the lines of the pressure- and percussion-figures, the other to the presence of cleavage rifts. The former possesses in a pronounced degree the property of asterism ; the latter, with which we are here dealing, shows pseudo-pleoehroism. Cleavage rifts and rod-like inclusions may both be present in considerable quantities, but pseudo-pleochroism is not seen when the latter are abundant.
Page 7 note 2 It can be readily reproduced by taking a plate of transparent muscovite and splitting it into a number of plates. When these are superposed, the pile will be found to be practically opaque to normally incident rays if the plates are numerous enough, though it is comparatively transparent to obliquely incident rays.
Page 8 note 1 This experiment can be performed with a few cover-slips mounted on a stage-goniometor ; but it is shown to best advantage with a piece of silvery mica sufficiently thick to be perceptibly translucent to rays of normal incidence. It is necessary that the surfaces and substance of the plates should be free from dust particles, &c. This can be seen by making a pile of cover-slips the surfaces of which have been slightly smoked. Such a pile scatters the refraction-polarized rays, and is less transparent to rays of oblique than it is to rays of normal incidence. A pile of celluloid plates acts in the same way, owing to the fact that celluloid is crammed with dusty inclusions.
Page 9 note 1 I am indebted to Professor G. A. J. Cole for calling my attention to a plate, of pink andalusite in his collection which appears to come under this case.
Page 11 note 1 ‘Textbook of Mineralogy,’ by E. S. Dana, 1908, p. 219 ; also ‘Mikroskopische Physiographie der petrographisch wichtigen Mineralien’, H. Rosenbusch and E. A. Wülfing, 1904, vol. i, part 1, p. 841. On this point see further remarks ia historical résumé, p. 25.
Page 14 note 1 Brewster, D., Phil. Trans., 1819, vol. cix, p. 17 Google Scholar.
Page 17 note 1 It is for this reason that absorption-brushes can often be seen by means of a nicol when they are imperceptible in ordinary light. In very thin or weakly pleoehroic plates the colour of the brushes becomes indistinguishable from the facial colour in ordinary light, though it may be quite perceptibly different from the extreme axial colours.
Page 17 note 2 For further remarks on these, see p. 19.
Page 17 note 3 This is the slightly ferriferous variety of phlogopite, showing a fairly pronounced axial angle, which occurs as a product of metamorphism in dolomitic limestones, &c. Even when the plates are not more than a millimetro in diameter, as often happens in these rocks, they are frequently pleochroic enough to show good absorption-figures under the microscope.
Page 18 note 1 Brewster described them in blue topaz and cordierite, in both of which the acute bisectrix is normal to the basal plane. The figure can be seen in ordinary brown Brazilian topaz, but only imperfectly, since owiug to the wide axial angle the optic axes emerge near the margin of the field ; and moreover, owing to the weakness of the pleochroism, it is necessary to use a nicol to see the figures clearly.
Page 19 note 1 The mirror of an ordinary microscope is unable to produce any appreciable degree of polarization. The polarization so often noticed in the light reflected from the metallic mirror of a microsoope is polarized skylight. It should also be noticed that some forms of artificial light are substantially polarized.
Page 19 note 2 According to Groth (‘Physikalische Krystallographie,’ 3rd edit., 1895, p. 159), andalusite shows the interference-rings in plates cut normal to an optia axis and examined in clear skylight ; but this is presumably true only when the skylight is polarized.
Page 20 note 1 Bertin, A., ‘Sur les houppes des cristaux polychroïques,’ Bull. Soc. Min. France, 1879, vol. ii, pp. 54–66 Google Scholar.
Page 20 note 2 A. Bertin, loc. cit., p. 66.
Page 20 note 3 Mallard, E., Bull. Soc. Min. France, 1879, vol. ii, p. 77 Google Scholar.
Page 22 note 1 Brewster, D., ‘On the phenomena of dichroism,’ Edinburgh Phil. Journ., 1820, vol. iii, p. 244 Google Scholar.
Page 23 note 1 Haidinger, W., Sitz. Akad. Win. Wien, 1854, vol. xiii, p. 812 Google Scholar.
Page 23 note 2 Brewster, D., Phil. Trans., 1819, vol. cix, p. 17 Google Scholar.
Page 24 note 1 See particularly ‘On the affections of light transmitted through crystallized bodies’, Phil. Trans., 1814, vol. civ, p. 203, where Brewster describes the interference-figure seen when ordinary light is sent obliquely through a plate of topaz, using an agate or a pile of plates as an analyser. I had already completed the observations embodied in the present paper before I found this record by Brewster, an earlier knowledge of which would have saved me much trouble. From the manuer in which Brewster made his observation on topaz, it does not follow that polarization by reflection and refraction was the cause of the figure he saw, unless lie took special precautions to see that there was no appreciable polarization in the ‘ordinary light’ which he used. His observation is, however, closely similar to that on colourless topaz which is given on p. 21 of the present paper.
Page 24 note 2 ‘Light,’ by Sir John F. W. Herschel; Encyclopedia Metropolitana, 1827, vol. iv, p. 557. The Encyclopedia as a whole was not published until 1845.
Page 25 note 1 J. F. W. Herschel, loc. cit., p. 562.
Page 25 note 2 Haidinger, W., ‘Ueber das directe Erkennen des polarisirten Lichts und der Lage der Polarisationsebene,’ Ann. Chem. Phys. (Poggendorff), 1844, vol. lxiii, pp. 29–89 CrossRefGoogle Scholar.
Page 25 note 3 Brewster, D., ‘On the coloured houppes or sectors of Haidinger,’ Phil. Mag., 1859, vol. xvii, p. 823 Google Scholar.
Page 25 note 4 See particularly Haidinger, W., ‘Pleochroismus an einigen zweiaxigen KrystaUen in neucrer Zeit beobachtet,’ Sitz. Akad. Wiss. Wien, 1854, vol. xiii, pp. 806–831 Google Scholar.
Page 25 note 5 See particularly Bertin, A., Bull. Soc. Min. France, 1879, vol. ii, p. 54 Google Scholar, followed by a discussion, to which Mallard. Bertrand, and others contributed. Also Bertin, A., ‘Sur les cristaux idiocyclophanes,’ Ann. Chim. Phys., 1878, ser. 5, vol. xv, pp. 396–430 Google Scholar.
Page 26 note 1 A. Bertin, loc. cir., p. 66.
Page 26 note 2 T. Liebisch, Nachr. Ges. Wiss. Göttingen, 1888. See also Liebisch's ‘Physikaliscbe Krystallographie’, 1891, and ‘Grundriss der physikalischen Krystallographie’, 1896.
Page 27 note 1 M. Tolstopiatow, ‘Notes sur les phénomènes des houppes,’ Recherches Minéraloglques, Édition Posthume, Moscou, 1893.
Page 27 note 2 Erman's ‘epoptic figures’ are cases of ‘idiocyclophany’. He was apparently unaware of Herschel's term, and of Brewster's earlier observations of similar effects.
Page 27 note 3 Erman, P., ‘Ueber die epoptischen Figuren des Arragonits ohne vorlôufige Polarisation,’ Ann. Chem. Phys. (Poggendorff), 1882, vol. xxvi, pp. 302–307 Google Scholar.
Page 27 note 4 Klein, C., ‘Die optischen Eigenschaften des Sulzbacher Epidots,’ Neues Jahrb. Min., 1874, pp. 1–21 Google Scholar.
Page 27 note 5 Voigt, W., ‘Erklärung der Farbenerscheinungen pleochroïtiseher Krystalle,’ Neues Jahrb. Min., 1885, vol. i, pp. 119–141 Google Scholar.
Page 27 note 6 W. Voigt, loc. cit.
Page 27 note 7 W. Voigt, Nachr. Ges. Wiss. Göttingen, 1902, p. 48.
Page 27 note 8 Voigt, W. ‘On the behaviour of pleochroitic crystals along directions in the neighbourhood of an optic axis,’ Phil. Mag., 1902, ser. 6, vol. iv, p. 90 CrossRefGoogle Scholar.
Page 28 note 1 Voigt, W., Phil. Mag., 1902, ser. 6, vol. iv, p. 96 Google Scholar.
Page 28 note 2 von Groth, p., ‘Physikalische Krystallographie,’ 8rd edit., 1895, p. 398 Google Scholar.
Page 28 note 3 E. S. Dana apparently describes absorption-figures as idiophanous figures (‘System of Mineralogy,’ 6th edit., 1892, pp. 420, 497, and 518. Also ‘Textbook of Mineralogy’, by E. S. Dana, 1908, p. 219); but there seems to be no justification for the use of the phrase ‘idiophanous figures’ in this sense.
