photograph of a portion of the solar spectrum in the middle, and the corresponding bright-line spectrum of iron each side of it. It will be seen that the coincidence of bright and dark lines is perfect.

481. Doppler's principle applied to light waves. We have seen (see The Doppler effect, § 387, p. 326) that the effect of the motion of a sounding body toward an observer is to shorten slightly the wave length of the note emitted, and the effect of motion away from an observer is to increase the wave length. Similarly, when a star is moving toward the earth, each particular wave length emitted will be slightly less than the wave length of the corresponding light from a source on the earth's surface. Hence in this star's spectrum all the lines will be displaced slightly toward the violet end of the spectrum. If a star is moving away from the earth, all its lines will be displaced toward the red end. From the direction and amount of displacement, therefore, we can calculate the velocity with which a star is moving toward or receding from the solar system. Observations of this sort have shown that some stars are moving through space toward the solar system with a velocity of 150 miles per second, while others are moving away with almost equal velocities. The whole solar system appears to be sweeping through space with a velocity of about 12 miles per second; but even at this rate it would be at least 70,000 years before the earth would come into the neighborhood of the nearest star, even if it were moving directly toward it.

QUESTIONS AND PROBLEMS

1. From the table on page 403 calculate how many waves of red and of violet light there are to an inch.

2. In what part of the sky will a rainbow appear if it is formed in the early morning?

3. Why do we believe that there is sodium in the sun?

4. What sort of spectrum should moonlight give? (The moon has no atmosphere.)

5. If you were given a mixture of a number of salts, how would you proceed, with a Bunsen burner, a prism, and a slit, to determine whether or not there was any calcium in the mixture?

6. Draw a diagram of a slit, a prism, and a lens, so placed as to form a pure spectrum.

7. How can you show that the wave lengths of red and green lights are different, and how can you determine which one is the longer?

CHAPTER XXI

INVISIBLE RADIATIONS

RADIATION FROM A HOT BODY

482. Invisible portions of the spectrum. When a spectrum is photographed, the effect on the photographic plate is found to extend far beyond the limits of the shortest visible violet rays. These so-called ultra-violet rays have been photographed and measured at the Ryerson Physical Laboratory, University of Chicago, down to a wave length of .00000273

centimeter, which is only one fifteenth the wave length of the shortest violet waves.

The longest rays visible in the extreme red have a wave length of about .00008 centimeter, but delicate thermoscopes reveal a so-called infra-red portion of the spectrum, the investigation of which was carried, in 1912, by Rubens and von Baeyer of Berlin, to wave lengths as long as .03 centimeter, 400 times as long as the longest visible rays.

FIG. 450. The Crookes radiometer

The presence of these long heat rays may be detected by means of the radiometer (Fig. 450), an instrument perfected by E. F. Nichols at Dartmouth. In its common form it consists of a partially exhausted bulb, within which is a little aluminium wheel carrying four vanes blackened on one face and polished on the other. When the instrument is held in sunlight or before a lamp, the vanes rotate in such a way that the blackened faces always move away from the source of radiation, because they absorb ether waves better than do the polished faces, and thus become hotter. The heated air in contact with these faces then exerts a greater pressure against them than does the air in contact with the polished faces.

G

A still simpler way of studying these long heat waves was devised in 1912 by Trowbridge of Princeton. A rubber band AC (Fig. 451) a millimeter wide is stretched to double its length over a glass plate FGHI, and the thinnest possible glass staff ED, carrying a light mirror E about 2 millimeters square, is placed under the rubber band at its middle point B. When the spectrum is thrown upon the portion AB of the band, the change in its length produced by the heating causes ED to roll, and a spot of light reflected from E to the wall to shift its position by an amount proportional to the heating.

B

+

D

H

FIG. 451. A simple thermoscope

Let either the radiometer or the thermoscope described above be placed just beyond the red end of the spectrum. It will indicate the presence here of heat rays of even greater energy than those in the visible spectrum. Again, let a red-hot iron ball and one of the detectors be placed at conjugate foci of a large mirror (Fig. 452). The invisible heat rays will be found to be reflected and focused just as are light rays. Next let a flat bottle filled with water be inserted between the detector and any source of heat. It will be found that water, although transparent to light rays, absorbs nearly all of the infra-red rays. But if the water is replaced by carbon bisulphide, the infra-red rays will be freely transmitted,

even though the liquid is rendered opaque to light waves by dissolving iodine in it.

483. Radiation and temperature. All boddies, even such as are at ordinary temperatures, are continually

FIG. 452. Reflection of infra-red rays

radiating energy in the form of ether waves. This is proved by the fact that even if a body is placed in the best vacuum obtainable, it continually falls in temperature when surrounded by a colder body, -for example, liquid air. The ether waves emitted at ordinary temperatures are doubtless very long as compared with light waves. As the temperature is raised,

more and more of these long waves are emitted, but shorter and shorter waves are continually added. At about 525° C. the first visible waves, that is, those of a dull red color, begin to appear. From this temperature on, owing to the addition of shorter and shorter waves, the color changes continuously, first to orange, then to yellow, and, finally, between 800° C. and 1200° C., to white. In other words, all bodies get "red-hot" at about 525° C. and "white-hot" at from 800° C. to 1200° C.

Some idea of how rapidly the total radiation of ether waves increases with increase of temperature may be obtained from the fact that a hot platinum wire gives out thirty-six times as much light at 1400° C. as it does at 1000° C., although at the latter temperature it is already white-hot. The radi ations from a hot body are sometimes classified as heat rays, light rays, and chemical, or actinic, rays. The classification is, however, misleading, since all ether waves are heat waves in the sense that, when absorbed by matter, they produce heating effects, that is, molecular motions. Radiant heat is, then, the radiated energy of ether waves of any and all wave lengths.

484. Radiation and absorption. Although all substances begin to emit waves of a given wave length at approximately the same temperature, the total rate of emission of energy at a given temperature varies greatly with the nature of the radiating surface. In general, experiment shows that surfaces which are good absorbers of ether radiations are also good radiators. From this it follows that surfaces which are good reflectors, like the polished metals, must be poor radiators.

Thus, let two sheets of tin, 5 or 10 centimeters square, one brightly polished and the other covered on one side with lampblack, be placed in vertical planes about 10 centimeters apart, the lampblacked side of one facing the polished side of the other. Let a small ball be stuck with a bit of wax to the outer face of each. Then let a hot metal

plate or ball (Fig. 453) be held midway between the two. The wax on the tin with the blackened face will melt and its ball will fall first, showing that the lampblack ab

sorbs the heat rays faster than does the polished tin. Now let two blackened glass bulbs be connected, as in Fig. 454, through a U-tube containing colored water, and let a wellpolished tin can, one side of which has been blackened, be filled with boiling water and placed between them. The motion of the water in the U-tube

FIG. 453. Good re-
flectors are poor
absorbers

will show that the blackened side of the can is radiating heat much more rapidly than the other, although the two are at the same temperature.

QUESTIONS AND PROBLEMS

1. The atmosphere is transparent to most of the sun's rays. Why are the upper regions of the atmosphere so much colder than the lower regions?

2. When one is sitting in front of an open-grate fire, does he receive most heat by conduction, by convection, or by radiation?

3. Sunlight in coming to the eye travels a much longer air path at sunrise and sunset than it does at noon. Since the sun appears red or yellow at these times, what rays are absorbed most by the atmosphere?

4. Glass transmits all the visible waves, but does not transmit the long infra-red rays. From this fact explain the principle of the hotbed.

5. Which will be cooler on a hot day, a white hat or a black one? 6. Will tea cool more quickly in a polished or in a tarnished metal vessel?

7. Which emits the more red rays, a white-hot iron or the same iron when it is red-hot?

8. Liquid-air flasks and thermos bottles are double-walled glass vessels with a vacuum between the walls. Liquid air will keep many times longer if the glass walls are silvered than if they are not. Why? Why is the space between the walls evacuated?