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twenty times, by the addition of different quantities of white light, it carries the number of tints we are able to distinguish up as high as two millions. In this calculation no account is taken of the whole series of purples, or of colours which are very luminous or very dark, or mixed with much white light.

To the above we may add that interesting experiments on the sensitiveness of the eye to the different spectral colours have also been made by Charles Pierce, who found that the photometric susceptibility of the eye was the same for all colours. (See “ American Journal of Science and Arts,” April, 1877.)

With the aid of Vierordt's measurements previously given, and the determinations by the author of the spaces occupied by the different colours in the spectrum, a very interesting point can now be settled, viz. : we can ascertain in what proportions the different colours are present in white light. The amount of red light, for example, which is present will evidently be equal to the space which it occupies in the spectrum, multiplied by its luminosity, and the same will be true of all the other colours. The author constructed a curve representing Vierordt's results, and from this, taken in combination with his own determinations of the extent of the coloured spaces, obtained the following table :

TABLE SHOWING THE AMOUNTS OF COLOURED LIGHT IN 1,000 PARTS OF WHITE

SUNLIGHT.

Red......
Orange-red.
Orange ..
Orange-yellow
Yellow....
Greenish-yellow
Yellowish-green..
Green and blue-green.
Cyan-blue...

54
.140

80 .114

64 .206 121 134 32

Blue......
Ultramarine and blue-violet.
Violet.

40
20
5

1,000

Artists are in the habit of dividing up colours into warm and cold. Now, if we draw the dividing line so as to include among the warm colours red, orange-red, orange, orange-yellow, yellow, greenish-yellow, and yellowish-green, then in white light the total luminosity of the warm colours will be rather more than three times as great as that of the cold colours. If we exclude from the list of warm colours yellowish-green, then they will be only about twice as luminous as the cold. We shall make use of this table hereafter.

It may have seemed strange that the chrome-yellow paper previously mentioned reflected eighty per cent. of light (the reflecting power of white paper being 100), while the table just given states that white light contains only a little more than five per cent. of pure yellow light. It will, however, be shown in a future chapter that chrome-yellow really reflects not only the pure yellow rays, but also the orange-yellow and greenish-yellow, besides much of the red, orange, and green light. By mixture, all these colours finally make a yellow, as will be explained in Chapter X. The high luminosity of some of the other coloured papers is to be explained in a similar manner.

CHAPTER IV.

PRODUCTION OF COLOUR BY INTERFERENCE AND

POLARIZATION.

In Chapter II. we studied the spectral tints produced by a prism and by a grating ; these were found to be pure

and brilliant as well as very numerous, and consequently were adopted as standards for comparison. Most nearly allied to these central normal colours are those which are produced by the polarization of light. In this class we meet with a far greater variety of hues than is presented by the solar spectrum ; and, instead of a simple arrangement of delicately shading bands, we encounter an immense variety of chromatic combinations, sometimes worked out with exquisite beauty, but as often arranged in a strange fantastic manner, that suggests we have entered a new world of colour, which is ruled over by laws quite different from those to which we have been accustomed. And it is indeed 80 ; the tints and their arrangement depend on the geometrical laws which build up the crystal out of its molecules, and on the retardation which the waves of light experience in sweeping through them, so that in the colours of polarization we see, as it were, Nature's mathematical laws laying aside for a moment their stiff awkwardness, and gayly manifesting themselves in play.

The apparatus necessary for the study of these fascinating and often audacious colour-combinations is not necessarily complicated or very expensive. A simple form of polariscope is shown in Fig. 11. It consists merely of a plate of window-glass at P, which is so placed that the angle, a, is 33° as nearly as possible ; at N is a Nicol's prism, and at La plano-convex lens with a focal length of about an inch. The distance of the prism from the plate

Fig. 11.-Simple Polarizing Apparatus.

of glass is ten inches; the lens is removable at pleasure, the Nicol's prism is capable of revolving around its longer axis. If, now, light from a white cloud be reflected from the plate of glass toward the Nicol's prism, as indicated by the arrow, some of it will ordinarily traverse the prism and reach the eye at E; the prism should now be turned till this light is cut off, and the instrument is then ready for use. Thin slips of selenite or crystals of tartaric acid placed at S, so that they are magnified, display the colours of polarization very beautifully. The arrangement just described constitutes a simple polarizing microscope ; if a compound polarizing microscope can be obtained, it will be still more easy to study the colours and combination of colours presented by the crystals of many different salts. By dissolving a grain or two of the substance in a drop of water, and allowing it to crystallize out on a slip of glass, it is possible very easily to make objects suitable for examination.

Thin plates of selenite, obtained by removing successive layers with a penknife, answer admirably if we wish to study the phenomena of chromatic polarization in their simplest forms. It will often be found that nearly the whole plate presents a single unshaded hue, which bears a close resemblance to a patch of colour taken from some portion of the spectrum. But, however bright the colour may be, it is never free from an admixture of white light, and it is the constant presence of this foreign element which causes the colours of polarization to appear a little less intense than those of the spectrum. With plates which are somewhat thicker the proportion of white light increases, washing out the colour, till only a faint reminiscence of it is left. The more powerful tints, however, quite equal and probably surpass in purity—that is, in freedom from white light—the most intense sunset hues. Among these colours we find many shades of red and purple-red; all the redorange hues are represented, and the same is true of the other colours found in the spectrum.

he spectrum. Besides, there is a range of purples which bridges over the chasm between the violets and reds ; faint rose-tints are also present in abundance, and the same is true of the pale greens and bluishgreens. In addition to this, quite thin slips furnish a distinct set of tints which are peculiar in appearance, and which, when once seen, are never forgotten ; a singular tawny yellow will be noticed in combination with a bluishgrey ; the yellow, such as it is, shading into an orange nearly allied to it, and this again into a brick-red ; black and white will be associated with these subdued tints ; the general impression produced by these combinations being sombre if not dreary.

These slips of selenite furnish neither beautiful nor complicated patterns, the tints being for the most part arranged in parallel bands, with here and there angular patches, often 'in sharp contrast with the other masses of colour. There is no noticeable attempt at chromatic composition, except perhaps a little along fractured edges, where we frequently meet with pale grey or white deepening into a fox-coloured yellow, followed by a red-violet, brightening into a seagreen dashed with pure ultramarine, or changing suddenly

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