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into a full orange-yellow, after which may follow a broad field of purple. Just as often all the tints are pale, like those used on maps, with a narrow fringe on the edge, of rich variegated hues. The colour-combinations seldom rise into great beauty, though they often astonish and dazzle by their audacity and total disregard of all known laws of chromatic composition. The brilliancy and purity of these tints are so great, and they are laid on with such an unfaltering hand, that all these wild freaks are performed comparatively with impunity, and it is only when we proceed to make copies of these strange designs that we become fully aware of their peculiarities, and, from an artistic point of view, positive defects.

Crystals of tartaric acid present phenomena which are quite different: here the patterns are rich and often beautiful; the colour is full of gradation, touched on and retouched and wrought out with patience in delicate, complicated forms, which echo or faintly oppose the grand ruling ideas of the composition. We may have a wonderfully shaped mass radiating in curved lines over the entire field, tinted with soft grey and pale yellow, with here and there dashes of colour like the spots on a peacock's tail, glowing like coals of fire; all this being set off by very dark shades of olive-green, dark browns and greys. If the crystals are thin this is their appearance ; but as the thickness increases so does the brilliancy of the hues, which are sure to be well contrasted with large masses of deep shade. dations, the sharp contrasts, the brilliant and pale colours, the dark shadows and the wonderful forms, all combine to lend to these pictures a peculiar charm which is not wholly lost even in copies executed in ordinary pigments.

Common sugar, if allowed time to crystallize out slowly, furnishes appearances somewhat resembling the above, but the designs are more formal and less interesting. Crystals of nitrate of potash present appearances, again, which are totally unlike those above mentioned ; here we have a great

The soft granumber of delicately tinted threads of light; there will be purples and golden greens or dull olive-greens and carmines, woven together so closely as almost to produce a neutral tint, which will brighten suddenly and display combinations of purple-red with green, dashed here and there with pure ultramarine. These tinted threads of light will be disposed with regularity as though it had been intended to weave them into some wonderful cashmere-like pattern, and then warp and woof had been suddenly abandoned and forgotten.

It would be useless to multiply these descriptions—every salt has its own peculiarities and suggests its own train of fancies ; some glow like coloured gems with polished facets, or bristle with golden spears like the advancing ranks of two hosts in conflict, or suggest a rich vegetation made of gold and jewels and bathed in sunset hues. Artists who see these exhibitions for the first time are generally very much impressed by their strange beauty, and not unfrequently insist that their range of colour-conceptions has been enlarged. It has often seemed to the author that the cautious occasional study of some of these combinations might be useful to the decorator in suggesting new conceptions of the possibilities within his reach.

When polarized light is made to traverse crystals in the direction of their optic axes, phenomena of a different kind are presented. They were discovered in 1813 by Brewster, and, on account of their scientific interest and a certain beauty, have since then greatly attracted the attention of physicists and even of mathematicians. A series of rainbowlike hues, disposed in concentric circles, is seen on a white field ; a dark-grey cross is drawn across the gayly coloured circles, and, after dividing them in four quadrants, fades out in the surrounding white field. By a slight change in the adjustment of the apparatus, the grey cross can be made white; the rings then assume the complementary tints. Other crystals, again, furnish double sets of rings, the dark cross being shared by them jointly, or so altered in form as no longer to be recognizable.

These appearances have been considered by many physicists to be extraordinarily beautiful ; it is, however, to be suspected that in this case the judgment was swayed by other considerations than those of mere beauty. The rarity of the phenomenon, the difficulty of exhibiting it, the brilliant list of names identified with it, along with the insight it furnishes as to the molecular constitution of crystals, all combine to warp the judgment, and to seriously influence its final award. In point of fact, the formal nature of the figures, the constant repetition of the rainbow-tints in the same set order, which is that of the spectrum, both exclude the possibility of the charming colour-combinations so frequently presented by many salts when simply crystallized on a slip of glass. The cross and rings are not for a moment, in matter of beauty, to be compared with the appearances presented by crystals of tartaric acid.

Glass which has been heated and then suddenly cooled, or glass which is under strain, exhibits phenomena of colour closely related to the above ; we have as it were a set of distorted crosses and rings which sometimes lend themselves more kindly to the production of chromatic effects than is the case with the normal figures.

In ordinary life the colours of polarization are never seen ; the fairy world where they reign cannot be entered without other aid than the unassisted eye. This is not a matter for regret ; the purity of the hues and the audacious character of their combinations cause their gayety to appear strange and unnatural to eyes accustomed to the far more sombre hues appropriate to a world in which labour and trouble are such important and ever-present elements. The colours even of flowers have a thoughtful cast, when compared with those of polarization.

The colours which have just been considered are produced in a peculiar manner; the complete explanation is

long and tedious, and has for us no particular interest. The main idea, however, is this : white light is acted on in such a way that one of its constituents is suppressed ; the result is coloured light. For example, if we strike out from white light the yellow rays, what remains will produce on us the sensation of blue ; if we cut off the green rays, the remainder will appear purple. The reason of this will be more fully appreciated after a study of the facts presented in Chapters XI. and XII. To effect this sifting out of certain rays a polarizing apparatus is employed; when the crystals are removed from it, the colour instantly vanishes. Now, -it so happens that there is a class of natural objects capable of displaying exactly the same hues without the intervention of any piece of apparatus—all objects that fulfill a certain condition

may

be reckoned in this class ; it is merely that their thickness should be very small. Thin layers of water, air, glass, of metallic oxides, of organic substances, in fact of almost everything, display these colours. The most familiar example is furnished by a soap-bubble. When it first begins to grow, it is destitute of colour and perfectly transparent; it gives by reflection from its spherical surface a distorted image of the window, with the bars all curved, but no unusual hues are noticed till it has become somewhat enlarged.* Then faint greens and rose-tints begin to make their appearance, mingling uneasily together as if subjected to a constant stirring process. As the bubble expands and the film becomes more attenuated, the colours gain in brilliancy, and a set of magnificent blue and orange hues, purples, yellows and superb greens replaces the pale colours which marked the early stages, and by their changing flow and perpetual play fascinate the beholder. If the bubble has the rare fortune to live to a good old age, at its upper portion a different series of tints begins to be developed ; the tawny yellow, before mentioned, begins to be seen in irregular patches, floating around among the more brilliant hues, a sign that the attenuation has nearly reached its extreme limit ; but, if by some unusual chance it should be a Methuselah among bubbles, pale white and grey tints also are seen, after which it is sure to burst. A long-lived soapbubble displays every colour which can be produced by polarization. The thin film has a sifting action on white light, which in its final result is the same as in the case of the production of colour by polarized light : certain rays are struck out, and, as before, white light deprived of one of its constituents furnishes coloured light. This elimination is accomplished by the interference of the waves of light involved ; hence, colours produced in this way are called “interference colours." The colours of polarization are also just as truly interference colours, but they are not usually known under this name. From all this it follows that the colours produced by thin layers, or by very fine particles, always contain some white light, and consequently cannot quite rival in purity or intensity the spectral hues.

* It is not very uncommon to meet with paintings in which a bubble has been represented with window-bars on its surface, where nothing of the kind could have been visible. A friend has mentioned to the author four cases where different artists have introduced window-bars instead of sky and landscape, on the surfaces of bubbles which were in the open air.

The colours of polarization, as we have seen, are never met with outside of the laboratory. Nature, on the other hand, here and there with a sparing hand, displays in small quantity, and as a rarity, the colours of interference. They are used as a wonderful kind of jewelry in the adornment of many birds ; lavishly so in the case of the common peacock, where the breast and tail feathers in full sunshine display flashing, dazzling hues, which make our artificial ornaments appear pale and tame. In contemplating these astonishing hues, or those of that tiny winged jewel, the humming-bird, we are struck by the circumstance that they actually have a metallic brilliancy, which we in vain attempt to rival with our brightest pigments. To compete with them successfully, it is necessary to substitute a sur

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