656.37. Find whether the earth and star are approaching or receding from each other and with what velocity.

2. If a star is moving toward the earth with a velocity of 18 miles per second, find the per cent. of change in the wave lengths of its spectrum lines due to the motion.

COLORS OF BODIES.

908. Colors of Bodies.-The colors of natural objects are due either to light waves which they themselves emit, or to their power of reflecting or absorbing the light that falls upon them from some external source. The first class includes all bodies that are self-luminous in consequence of―

a. high temperature, as in red-hot or white-hot bodies,

b. chemical action, as in flames,

c. electric discharge,

d. stimulus of light from other sources, as in fluorescence and phosphorescence.

to

The second class includes bodies whose colors are due either

a. selective absorption, as in colored glass, pigments, and most colored bodies; or

b. selective reflection, as in metals and bodies showing special luster.

909. Luminous Bodies.-The color of the light from any source is the average effect of its radiation upon the eye; but the particular kind of radiation which causes the effect can be determined only by analyzing the light with a spectroscope.

For example, a yellow gas flame is found to have in its spectrum all kinds of light, but the blue and violet rays are relatively less intense than in sunlight. It is this weakness in the blue and violet which gives it a yellow color. On the other hand, the spectroscope shows that the sodium light or the yellow light obtained when a bit of common salt, previously fused, is held by a loop of platinum wire in the pale blue flame of a Bunsen burner, is yellow for an entirely different reason; for the spectrum of this light consists principally of two yellow lines so close together that they appear like one line in a spectroscope of low power. The light from this flame is therefore very nearly

homogeneous and appears yellow because the only kind of light present is one that excites that color sensation and no other.

910. Non-luminous Bodies.-Non-luminous bodies show no color in the dark. They derive their color from the light by which they are illuminated. Let sunlight fall on a piece of colored glass or a vessel containing some strongly colored dye. The light reflected from the surface of the glass or solution shows no trace of color, indicating that such substances reflect all kinds of light equally. But light passing through the glass or colored solution is deeply colored and when examined by the spectro

scope broad dark bands are seen in its spectrum, showing that certain constituents of sunlight have been strongly absorbed by the substance.

Thus a dark blue cobalt glass transmits green, blue,

FIG. 563.-Spectrum of light transmitted by and violet, but absorbs

blue cobalt glass.

strongly yellow and orange and most of the red. The curve in figure 563 represents by its height the intensity, in different parts of the spectrum, of light which has passed through a certain thickness of this kind of glass. Such absorption is called selective.

911. Spectrophotometer.-An instrument in which the spectra from two sources are formed side by side and with appliances so that the relative intensities of the two spectra can be determined for each point in the spectrum is known as a spectrophotometer.

Such a curve as that shown in figure 563 is obtained by means of an instrument of this kind, the spectrum of direct sunlight being compared with that of sunlight which has passed through the colored substance.

912. Absorption and Color of Powders.-The most common cause of the color of bodies is absorption. A crystal of copper sulphate when seen by ordinary daylight appears blue because light coming to the eye through the crystal has lost the red and yellow rays by absorption. The light received by the eye is, however, not a pure blue, but is diluted with white light reflected from its surface. If the crystal is broken into smaller frag

ments the thickness of crystal through which the transmitted light passes before meeting a reflecting surface is smaller and there is accordingly less absorption and the blue color is not so marked. If the crystal is finely pulverized, the dry powder appears a pale whitish-blue, for light can penetrate only to an extremely small depth before being reflected and scattered by the surfaces of the tiny fragments.

From the above considerations it is evident that all clear colorless substances, such as ice, glass, Iceland spar, etc., must make white powders, since they reflect and scatter the light from innumerable minute surfaces but absorb scarcely any of the visible rays. The light reflected to the eye by such a powder is, therefore, of the same quality as that which falls upon it, and when illuminated by white light it appears white.

913. Effect of Illumination.-Except in case of self-luminous bodies, the color of a substance depends on the light by which it is illuminated. When a piece of red paper is held in the red of a bright spectrum it appears bright red, but when held in the yellow, green, or blue parts of the spectrum it appears black, for it can reflect red rays, but it absorbs the yellow, green and blue. So a blue paper may reflect the violet, blue and green, but will appear black in the red, orange, or yellow parts of the spectrum, while a white paper reflects whatever color falls upon it.

Two kinds of light that appear very much alike in color may yet have very different effects on the colors of bodies. For example, an ordinary gas flame gives out a yellowish light not very unlike the yellow sodium flame in appearance, and yet bright-colored objects or pieces of paper are seen in their various colors when illuminated by the gas flame, but all appear of one color, either brighter or darker yellow or black, when illuminated with the sodium flame. This is because the ordinary flame gives out all kinds of light waves from red to violet, while the sodium light is nearly homogeneous. The peculiar, ghastly appearance of persons illuminated by a salted alcohol flame is due to this cause.

The difference between colors seen by daylight and gaslight is because light from the blue end of the spectrum is relatively far more intense in the former than in the latter.

914. Matched Colors.-Two colors that appear alike by daylight may yet be due to very different kinds of light. When

an object appears yellow it does not follow that it reflects only rays from the yellow part of the spectrum. It means simply that the stimulus given to the retina by the various kinds of rays coming from the object excites the same sensation as the yellow light of the spectrum. What particular waves cause the color can be determined only by dispersing the light and examining its spectrum.

Consequently two colors which match perfectly when seen by daylight may differ very much when illuminated by some artificial light.

915. Mixed Pigments.-When paints are mixed the resulting color is not a mixture of the colors that each would give separately, but is due to the double absorption which light suffers in the mixture. For instance, a solution of gamboge yellow absorbs

all rays but yellow and green, while a solution of Prussian blue absorbs all but blue and green, a mixture of the two will therefore transmit only the green.

916. Mixing Colors.-To find the color that will be produced by a mixture or blending of colored lights, the lights to be mixed may be made to illuminate simultaneously a white screen, or the color wheel may be employed. In this apparatus discs of colored paper, each slit from center to edge, and fitted together so as to expose a sector of each color, are mounted on a spindle and rapidly rotated. If the speed of rotation is sufficient the disc appears of a uniform color which is the mixture of the different colors used. The proportional amounts of the several colors depends on the widths of the exposed sectors and may be changed by slipping the discs on each other. A larger and smaller disc may be mounted on the same spindle for comparison.

The effect in this case depends on the persistence of the sensation for a very short time after the stimulus to the retina has

ceased. The various colors give their stimuli in such rapid succession that the effect is a blended sensation.

Newton found that a color disc painted in sectors to imitate the colors of the spectrum appeared grayish-white when rapidly rotated and could be matched with a black disc having a white sector, black being used to diminish the intensity of the white.

Complementary Colors.-Two colors which when combined produce white, are said to be complementary. By means of the color disc it is found that blue and yellow of the proper tints and intensities will make white, also green and red may be complementary.

917. Metallic Luster.-Metals owe their peculiar luster to their intense reflecting power. Polished silver reflects 90 per cent. of the light that falls upon it, while glass at perpendicular incidence reflects less than 5 per cent.

Sunlight reflected from red or blue glass remains white, but when reflected from gold-leaf it is yellow. This shows that the reflection of light in case of some metals is selective, some kinds of light being more strongly reflected than others. It is to this property that the colors of metals are due.

The light transmitted through a thin film of gold-leaf is not yellow, but green. The yellow light which is reflected is that also for which the absorbing power of the metal is greatest.

Some non-metallic substances also have the power of reflecting light like metals as is seen in the bronzy luster of aniline ink and in crystals of permanganate of potash. Such substances show strong selective absorption and anomalous dispersion.

918. Fluorescence. When a strong beam of sunlight or light from the electric arc is sent through a block of glass colored with oxide of uranium the transmitted light is yellowish, showing that there has been absorption of the shorter wave lengths; but besides this, the whole block of glass is seen to glow with a greenish light which seems to come from each point in the glass itself, making the whole block seem turbid and milky. This is called fluorescence, for the phenomenon is strongly marked in fluorspar.

The subject was first carefully investigated by Sir George Stokes, who showed that fluorescence is really a kind of radiation from the molecules of the substance under the stimulus of the ab