riable position are used as standards of wave length. A careful comparison of the position of these lines with the bright line spectra of iron, copper, silver, zinc, sodium, and many other elements shows that they have exactly the same wave lengths, and hence it is concluded that these elements exist

in the atmosphere of the sun in a state of vapor. The positions of the most prominent of the Fraunhofer lines are shown in Fig. 514, in which the violet end of the spectrum is at the right.

The above table gives the wave lengths of some of the dark lines of the spectrum. There are parts of the visible spectrum in the red end that have longer wave lengths than the A line, as well as parts in the violet that have shorter wave lengths than the H line.

548. The Invisible Spectrum. The visible spectrum is by no means the limit of the dispersion secured by the

prism. Beyond the red are a series of longer waves that can easily be detected as heat rays, while beyond the violet are shorter waves that have great chemical activity, and are called the chemical or actinic rays.

Demonstration.-Cut a piece of photographic printing paper into strips and pin them end to end upon the wall of a dark room so as to form a long strip. By means of a prism throw the solar spectrum upon the middle of the strip, which should extend a third of its length beyond each end of the spectrum. Mark the position of every color and the end of the visible spectrum. After a short exposure notice that the greatest effect on the paper is at the violet end of the spectrum or just beyond it, and that at the red end there is practically no change.

The 549. Doppler's Principle Applied to Light Waves. position of the dark lines of the spectrum is invariable only when the distance between the source of light and the observer is fixed. If this distance is regularly diminishing, the wave length corresponding to a given line diminishes, and if it is increasing, the wave length increases.

R

C

V

If the spectrum of a star is examined, and it is found that the C line, for instance, is located at the right of the position of that line in the solar spectrum, as the lighter line in Fig. 515, the explanation is that the

FIG. 515

star is moving toward us and thus shortening the wave length that produces the line. If the displacement is toward the red end of the spectrum, then the star is receding from us. Since the velocity of the star determines the amount of this displacement, a measurement of this amount can be used as a basis for calculating the velocity of the star's motion.

550. Interference. We have already seen the results of interference in sound waves. Two waves may meet in such a way as to strengthen each other or to neutralize each other. Similar phenomena occur in light.

Demonstration. — Against a piece of plate glass press the curved side of a plano-convex lens of great focal length. On looking at the upper surface of the lens at an angle, a series of concentric circles will be seen, each one of which will be made up of the colors of the spectrum. If a sheet of red glass is placed between the lens and the light, so that light of that color alone falls upon the lens, the rings will be alternate dark and red bands. Two strips of plate glass separated at one end by a sheet of paper and pinched together at the other will also show interference bands.

The rings shown by the lens are called Newton's rings. The colors seen on looking at a thin film of oil on water, a soap bubble, or a crack in a piece of ice, are other examples of interference.

551. Explanation of Newton's Rings. When light strikes the lens the refracted ray is partly reflected at A and its phase

length, the reflected rays are in the same phase and give light; but if it is a half wave length the reflected rays are in

1 A change of phase of half a wave length takes place whenever light is reflected back into a dense medium instead of passing into

a rarer one.

opposite phases and there is interference or darkness. As the space between the curved surface of the lens and the plane glass increases gradually, there is a distance that corresponds to a quarter wave length for every color of the spectrum.

552. Diffraction. -Demonstrations. -Expose a photographic dry plate to daylight. Develop it and fix it. Wash it thoroughly and dry it. With a fine needle or the point of a knife blade draw a series of parallel lines through the film to the glass. Hold this plate close to the eye and look through it at the flame of a candle. There will be seen brilliant spectrum colors extending on either side of the flame.

On one half of the plate used above, draw two sets of parallel lines at right angles to each other, dividing the film into small squares. Look through this at an arc or incandescent light, and fine lines of spectra will be seen to extend at right angles from the light.

A fine silk handkerchief gives a good effect when looked through, especially when the light examined is an arc light. A plate of glass upon which a little lycopodium powder has been sprinkled gives a series of beautiful rainbow effects when a candle or any bright light is seen through it. The cause of these phenomena is that rays of light on passing through a narrow slit spread out into a diverging band, or are diffracted; and if the edges of the slit are near each other, interference takes place and gives colored fringes.

The diffraction grating consists of from 10,000 to 20,000 parallel lines to the inch, ruled on glass or on speculum metal. If on the former, the light passes through, if on the latter, it is reflected from the polished and ruled surface. Spectra of wide dispersion can be obtained from such gratings, and they are used in place of the prism in spectroscopic work. The spectrum given by the grating is called the normal spectrum, since in it the distribution of the different wave lengths is uniform.

553. Polarized Light. The vibrations of the ether that produce a ray of light are transverse vibrations in every

transversely, as in Fig. 518, then the ray would be a ray of plane polarized light. Polarized light affects the eye just like ordinary light, but it presents certain very interesting phenomena, some of which are shown in the following pages.

The crystal tourmaline has the property of permitting vibrations in only one plane to emerge from

it; that is, it polarizes light.

FIG. 519

Demonstrations. Place two tourmaline crystals one over the other and parallel to each other. The light that passes through one will pass through the other. Now turn one of them until it crosses the other at right angles, as in Fig. 519, and no light at all passes through the crossed portion. The action is as though each let through only those vibrations which are parallel to its length. Now put between the tourmalines a piece of quartz or Iceland spar, and on turning one of the tourmalines, brilliant color effects are observed. The effect of the quartz, then, must be to turn the plane of polarization and to enable a part of the light to pass through the second tourmaline, producing the color effects by partial interference of the polarized rays.

Lay a sheet of black paper upon the table. Over this lay a sheet of glass G (Fig. 520). Cut out ten or twelve pieces of thin glass, and holding them as at A in the figure, look through them at a piece of mica M laid upon the glass sheet. Hold the thin glasses at