Set a small light near one edge of a mirror; then, by putting the eye near the opposite edge, you see almost as many flames as you please from the multiplied reflections. How can this be accounted for?

Into your beam of sunlight, admitted through a half-inch hole, put the mirror at an oblique angle; you can arrange it so as to throw half a dozen bright spots on the opposite wall.

[Ill.u.s.tration: Fig. 10.--Manifold Reflections.]

In Fig. 10 the sunbeam enters at A, and, striking the mirror _m_ at _a_, is partly reflected to 1 on the wall, and partly enters the gla.s.s, pa.s.ses through to the silvered back at B, and is totally reflected to _b_, where it again divides, some of it going to the wall at 2, and the rest, continuing to make the same reflections and divisions, causes spots 3, 4, 5, etc. The brightest spot is at No.2, because the silvered gla.s.s at B is the best reflector and has the most light.

When the discovery of the moons of Mars was announced in 1877, it was also widely published that they could be seen by a mirror.

Of course this is impossible. The point of light mistaken for the moon in this secondary reflection was caused by holding the mirror in an oblique position.

Take a small piece of mirror, say an inch in surface, and putting under it three little pellets of wax, putty, or clay, set it on the wrist, with one of the pellets on the pulse. Hold the mirror steadily in the beam of light, and the frequency and prominence of each pulse-beat will be indicated by the tossing spot of light on the wall. If the operator becomes excited the fact will be evident to all observers.

[Ill.u.s.tration: Fig. 11.]

Place a coin in a basin (Fig. 11), and set it so that the rim will conceal the coin from the eye. Pour in water, and the coin will [Page 40] appear to rise into sight. When light pa.s.ses from a medium of one density to a medium of another, its direction is changed.

Thus a stick in water seems bent. Ships below the horizon are sometimes seen above, because of the different density of the layers of air.

Thus light coming from the interstellar s.p.a.ces, and entering our atmosphere, is bent down more and more by its increasing density.

The effect is greatest when the sun or star is near the horizon, none at all in the zenith. This brings the object into view before it is risen. Allowance for this displacement is made in all delicate astronomical observations.

[Ill.u.s.tration: Fig. 12.--Atmospherical Refraction.]

Notice on the floor the shadow of the window-frames. The gla.s.s of almost every window is so bent as to turn the sunlight aside enough to obliterate some of the shadows or increase their thickness.

DECOMPOSITION OF LIGHT.

Admit the sunbeam through a slit one inch long and one-twentieth of an inch wide. Pa.s.s it through a prism. Either purchase one or make it of three plain pieces of gla.s.s one and a half inch wide by six inches long, fastened together in triangular shape--fasten the edges with hot wax and fill it with water; then on a screen or wall you will have the colors of the rainbow, not merely seven but seventy, if your eyes are sharp enough.

Take a bit of red paper that matches the red color of the spectrum.

Move it along the line of colors toward the violet. In the orange it is dark, in the yellow darker, in the green and all beyond, black. That is because there are no more red rays to be reflected by it. So a green object is true to its color only in the green rays, and black elsewhere. All these colors may be recombined by a second prism into white light.

[Page 41]

III.

ASTRONOMICAL INSTRUMENTS.

"The eyes of the Lord are in every place."--_Proverbs_ xv. 3.

[Page 42]

"Man, having one kind of an eye given him by his Maker, proceeds to construct two other kinds. He makes one that magnifies invisible objects thousands of times, so that a dull razor-edge appears as thick as three fingers, until the amazing beauty of color and form in infinitesimal objects is entrancingly apparent, and he knows that G.o.d's care of least things is infinite. Then he makes the other kind four or six feet in diameter, and penetrates the immensities of s.p.a.ce thousands of times beyond where his natural eye can pierce, until he sees that G.o.d's immensities of worlds are infinite also."--BISHOP FOSTER.

[Page 43]

III.

_THE TELESCOPE._

Frequent allusion has been made in the previous chapter to discovered results. It is necessary to understand more clearly the process by which such results have been obtained. Some astronomical instruments are of the simplest character, some most delicate and complex.

When a man smokes a piece of gla.s.s, in order to see an eclipse of the sun, he makes a simple instrument. Ferguson, lying on his back and slipping beads on a string at a certain distance above his eye, measured the relative distances of the stars. The use of more complex instruments commenced when Galileo applied the telescope to the heavens. He cannot be said to have invented the telescope, but he certainly constructed his own without a pattern, and used it to good purpose. It consists of a lens, O B (Fig. 13), which acts as a multiple prism to bend all the rays to one point at R. Place the eye there, and it receives as much light as if it were as large as the lens...o...b.. The rays, however, are convergent, and the point difficult to [Page 44] find. Hence there is placed at R a concave lens, pa.s.sing through which the rays emerge in parallel lines, and are received by the eye. Opera-gla.s.ses are made upon precisely this principle to-day, because they can be made conveniently short.

[Ill.u.s.tration: Fig. 13.--Refracting Telescope.]

If, instead of a concave lens at R, converting the converging rays into parallel ones, we place a convex or magnifying lens, the minute image is enlarged as much as an object seems diminished when the telescope is reversed. This is the grand principle of the refracting telescope. Difficulties innumerable arise as we attempt to enlarge the instruments. These have been overcome, one after another, until it is now felt that the best modern telescope, with an object lens of twenty-six inches, has fully reached the limit of optical power.

_The Reflecting Telescope_.

This is the only kind of instrument differing radically from the refracting one already described. It receives the light in a concave mirror, M (Fig. 14), which reflects it to the focus F, producing the same result as the lens of the refracting telescope. Here a mirror may be placed obliquely, reflecting the image at right angles to the eye, outside the tube, in which case it is called the Newtonian telescope; or a mirror at R may be placed perpendicularly, and send the rays through [Page 45] an opening in the mirror at M. This form is called the Gregorian telescope. Or the mirror M may be slightly inclined to the coming rays, so as to bring the point F entirely outside the tube, in which case it is called the Herschelian telescope. In either case the image may be magnified, as in the refracting telescope.

[Ill.u.s.tration: Fig. 14.--Reflecting Telescope.]

Reflecting telescopes are made of all sizes, up to the Cyclopean eye of the one constructed by Lord Rosse, which is six feet in diameter. The form of instrument to be preferred depends on the use to which it is to be put. The loss of light in pa.s.sing through gla.s.s lenses is about two-tenths. The loss by reflection is often one-half. In view of this peculiarity and many others, it is held that a twenty-six-inch refractor is fully equal to any six-foot reflector.

The mounting of large telescopes demands the highest engineering ability. The whole instrument, with its vast weight of a twenty-six-inch gla.s.s lens, with its accompanying tube and appurtenances, must be pointed as nicely as a rifle, and held as steadily as the axis of the globe. To give it the required steadiness, the foundation on which it is placed is sunk deep in the earth, far from rail or other roads, and no part of the observatory is allowed to touch this support. When a star is once found, the earth swiftly rotates the telescope away from it, and it pa.s.ses out of the field. To avoid this, clock-work is so arranged that the great telescope follows the star by the hour, if required. It will take a star at its eastern rising, and hold it constantly in view while it climbs to the meridian and sinks in the west (Fig. 15). The reflector demands still more difficult engineering. That of Lord Rosse has a metallic mirror [Page 46] weighing six tons, a tube forty feet long, which, with its appurtenances, weighs seven tons more. It moves between two walls only 10 east and west. The new Paris reflector (Fig. 16) has a much wider range of movement.

[Ill.u.s.tration: Fig. 15.--Cambridge Equatorial.]

[Ill.u.s.tration: Fig. 16.--New Paris Reflector.]

_The Spectroscope._

A spectrum is a collection of the colors which are dispersed by a prism from any given light. If it is sunlight, it is a solar spectrum; if the source of light is a [Page 49] star, candle, glowing metal, or gas, it is the spectrum of a star, candle, glowing metal, or gas. An instrument to see these spectra is called a spectroscope. Considering the infinite variety of light, and its easy modification and absorption, we should expect an immense number of spectra. A mere prism disperses the light so imperfectly that different orders of vibrations, perceived as colors, are mingled. No eye can tell where one commences or ends. Such a spectrum is said to be impure. What we want is that each point in the spectrum should be made of rays of the same number of vibrations. As we can let only a small beam of light pa.s.s through the prism, in studying celestial objects with a telescope and spectroscope we must, in every instance, contract the aperture of the instrument until we get only a small beam of light. In order to have the colors thoroughly dispersed, the best instruments pa.s.s the beam of light through a series of prisms called a battery, each one spreading farther the colors which the previous ones had spread. In Fig. 17 the ray is seen entering through the telescope A, which renders the rays parallel, and pa.s.sing [Page 50] through the prisms out to telescope B, where the spectrum can be examined on the retina of the eye for a screen. In order to still farther disperse the rays, some batteries receive the ray from the last prism at O upon an oblique mirror, send it up a little to another, which delivers it again to the prism to make its journey back again through them all, and come out to be examined just above where it entered the first prism.

[Ill.u.s.tration: Fig. 17.--Spectroscope, with Battery of Prisms.]

Attached to the examining telescope is a diamond-ruled scale of gla.s.s, enabling us to fix the position of any line with great exactness.

[Ill.u.s.tration: Fig. 18.--Spectra of glowing Hydrogen and the Sun.]

In Fig. 18 is seen, in the lower part, a spectrum of the sun, with about a score of its thousands of lines made evident. In the upper part is seen the spectrum of bright lines given by glowing hydrogen gas. These lines are given by no other known gas; they are its autograph. It is readily observed that they precisely correspond with certain dark lines in the solar spectrum. Hence we easily know that a glowing gas gives the same bright lines that it absorbs from the light of another source pa.s.sing through it--that is, glowing gas gives out the same rays of light that it absorbs when it is not glowing.

The subject becomes clearer by a study of the chromolithic plate.

No. 1 represents the solar spectrum, with a few of its lines on an accurately graduated scale. [Page 51] No.3 shows the bright line of glowing sodium, and, corresponding to a dark line in the solar spectrum, shows the presence of salt in that body. No. 2 shows that pota.s.sium has some violet rays, but not all; and there being no dark line to correspond in the solar spectrum, we infer its absence from the sun. No.6 shows the numerous lines and bands of barium--several red, orange, yellow, and four are very bright green ones. The lines given by any volatilized substances are always in the same place on the scale.

A patient study of these signs of substances reveals, richer results than a study of the cuniform characters engraved on a.s.syrian slabs; for one is the handwriting of men, the other the handwriting of G.o.d.

One of the most difficult and delicate problems solved by the spectroscope is the approach or departure of a light-giving body in the line of sight. Stand before a locomotive a mile away, you cannot tell whether it approaches or recedes, yet it will dash by in a minute. How can the movements of the stars be comprehended when they are at such an immeasurable distance?

It can best be ill.u.s.trated by music. The note C of the G clef is made by two hundred and fifty-seven vibrations of air per second.

Twice as many vibrations per second would give us the note C an octave above. Sound travels at the rate of three hundred and sixty-four yards per second. If the source of these two hundred and fifty-seven vibrations could approach us at three hundred and sixty-four yards per second, it is obvious that twice as many waves would be put into a given s.p.a.ce, and we should hear the upper C when only waves enough were made for the lower C. The same [Page 52] result would appear if we carried our ear toward the sound fast enough to take up twice as many valves as though we stood still. This is apparent to every observer in a railway train. The whistle of an approaching locomotive gives one tone; it pa.s.ses, and we instantly detect another. Let two trains, running at a speed of thirty-six yards a second, approach each other. Let the whistle of one sound the note E, three hundred and twenty-three vibrations per second. It will be heard on the other as the note G, three hundred and eighty-eight vibrations per second; for the speed of each train crowds the vibrations into one-tenth less room, adding 32+ vibrations per second, making three hundred and eighty-eight in all. The trains pa.s.s. The vibrations are put into one-tenth more s.p.a.ce by the whistle making them, and the other train allows only nine-tenths of what there are to overtake the ear. Each subtracts 32+ vibrations from three hundred and twenty-three, leaving only two hundred and fifty-eight, which is the note C. Yet the note E was constantly uttered.

[Ill.u.s.tration: 1. Solar Spectrum. 2. Spectrum of Pota.s.sium. 3.

Spectrum of Sodium. 4. Spectrum of Strontium. 5. Spectrum of Calcium.

6. Spectrum of Barium.]