face of silver for the white paper, and to cover it with the purest and most transparent glazes. This appearance of metallic lustre depends on the circumstance that much coloured light is reflected, mingled with only a small quantity of white light, the great bulk of the latter being absorbed by the dark pigment contained in the interior of the feathers. When this dark pigment is absent, we have as before colour; but, being mingled with much reflected white light, it presents simply an appearance like that of mother-ofpearl.

There is yet another peculiarity of the colours now under consideration, which still more completely separates them from the hues furnished by pigments: it is their variability. These colours, as has been mentioned, are produced by the interference of the waves of light which are reflected from the thin films: the nature of this interference depends partly on the angle at which this reflection takes place, so that, as we turn a peacock's feather in the hand, its colour constantly changes. The same is true of the tints of the soap-bubble, and of interference colours in general -the hue changes with the position of the eye; as they are viewed more and more obliquely, the tint changes in the order of the spectrum, viz., from red to orange, to yellow,

etc.

The brilliant metallic colours exhibited by many insects, particularly the beetles, belong also in this class, so also the more subdued steel-blues and bottle-greens displayed by many species of flies. So commonly does this occur that it suggests the idea that these humble creatures are not destitute of a sense for colour capable of gratification by brilliant hues. If we descend into the watery regions we find their inhabitants richly decorated with colours of the same general origin, the pearly rainbow hues which they display all depending on the interference of light. The same is true of the iridescent hues which so commonly adorn shells externally and internally. In this case candour compels one

to admit that the colours, beautiful as they are, can hardly be a source of pleasure to the occupants or to their friends.

Leaving the animated world, we find the colours of interference shown frequently, but in an inconspicuous manner, by rather old window-glass; some of the alkali seems to be removed by the rain, and in the course of time a thin film of silica capable of generating these hues is formed. In antique glass which has long remained buried this process is carried much further, so that sometimes the whole plate or vase tends to split up into flakes. Here, owing to successive reflections on many layers, the light which reaches the eye is quite bright, and the colours intense. Crimson, azure, and gold are found in combination; blue melts into purple or flashes into red; ruby tints contrast with emerald hues each change of the position of the eye or of the direction of the light gives rise to a new and startling effect. In other cases broad fields of colour, with much gentle gradation and mingling of tender pearly hues, replace the gorgeous prismatic tints, and fascinate the beholder with their soft brilliancy.

The iridescent hues of many minerals fall into the same general class; they are beautifully displayed by some of the feldspars, and the brilliant hues found on anthracite coal have also the same origin. The blue films often purposely produced on steel are due to thin layers of oxide of iron which suppress the yellow rays. Other cases might be mentioned, but these will suffice for the present.

CHAPTER V.

ON THE COLOURS OF OPALESCENT MEDIA.

IF white light be allowed to fall on water which is contained in a clear, colourless glass vessel, some of it will be reflected from the surface of the liquid, while another portion will traverse the water and finally again reach the air. These well-known facts are represented in Fig. 12. An eye placed at E will perceive the reflected light to be white, and the transmitted light will also appear white to an eye situated at O. But, if now a little milk be added to the water, a remarkable change will be produced: light will, as before, be reflected from the surface to the eye placed at E, and this surface-light will still be white; but the little milkglobules under the surface and throughout the liquid will also reflect light to E—this light will be bluish. From this experiment, then, it appears that the minute globules suspended in the liquid have the power of reflecting light of a bluish tint. In Fig. 12 the light is represented as being reflected only in one direction; but, when the milk-globules are added, they scatter reflected light in many directions, so that an eye placed anywhere above the liquid perceives this bluish appearance.

On the other hand, after the addition of the milk, the light at O (Fig. 12), which has passed through the milky liquid, will be found to have acquired a yellowish tint. From this it appears that fine particles suspended in a liquid have the power of dividing white light into two portions, tinted respectively yellowish and bluish. If more milk be

added to the water, white light will mingle in and will finally overpower the bluish reflected light, so that it will hardly be noticed; as the quantity of milk is increased, the colour of the transmitted light will pass from yellow to orange, to red, and finally disappear, the liquid having become at last so opaque as to cease to transmit light altogether.

This very curious action is not confined to mixtures of milk and water, but is exhibited whenever very fine particles are suspended in a medium different from themselves. If an alcoholic solution of a resin is poured with constant stirring into water, very fine particles of resin are left suspended in the liquid, and give rise to the appearances just described. Brücke dissolves one part of mastic in eightyseven parts of alcohol, and then mixes with water, the water

being kept in constant agitation. A liquid prepared in this way shows by reflected light a soft sky-like hue, the colour of the light which has passed through being either yellow or red, according to the thickness of the layer traversed. The suspended particles of resin are very fine, and remain mingled with the water for months; they are often so fine as to escape detection by the most powerful microscopes.

Some kinds of glass which are used for ornamental purposes possess the same property, appearing bluish-white by reflected light, but tingeing the light which comes through them red or orange-red. The beautiful tints of the opal probably have the same origin, and the same is true also of the bluish, milky colour which characterizes many other varieties of quartz.

Not only liquids and solids exhibit this phenomenon of opalescence, but we find it also sometimes displayed elsewhere; thus, for example, a thin column of smoke from burning wood reflects quite a proportion of blue light, while the sunlight which traverses it is tinted of a brownish-yellow, or it may be, even red, if the smoke is pretty dense.

All these phenomena are probably due to an interference of light, which is brought about by the presence of the fine particles, the shorter waves being reflected more copiously than those which are longer; these last, on the other hand, being more abundantly transmitted. An elaborate explanation of the mode in which the interference takes place would be foreign to the purpose of the present work; we therefore pass on to the consideration of the more practical aspects of this matter.*

It will be well to notice, in the first place, certain conditions which favour not so much the formation as the perception of the tints in question: thus it will be found that

* Compare E. Brücke, in Poggendorff's "Annalen," Bd. 88, S. 363 ; also Bezold's "Farbenlehre," p. 89.