[Ill.u.s.tration: FIG. 125.]

THE GALILEAN TELESCOPE.

[Ill.u.s.tration: FIG. 126.]

A third form of telescope is that invented by the great Italian astronomer, Galileo,[24] in 1609. Its principle is shown in Fig. 126.

The rays transmitted by the object-gla.s.s are caught, _before_ coming to a focus, on a concave lens which separates them so that they appear to meet in the paths of convergence denoted by the dotted lines. The image is erect. Opera-gla.s.ses are constructed on the Galilean principle.

THE PRISMATIC TELESCOPE.

In order to be able to use a long-focus object-gla.s.s without a long focussing-tube, a system of gla.s.s reflecting prisms is sometimes employed, as in Fig. 127. A ray pa.s.sing through the object-gla.s.s is reflected from one posterior surface of prism A on to the other posterior surface, and by it out through the front on to a second prism arranged at right angles to it, which pa.s.ses the ray on to the compound eye-piece. The distance between object-gla.s.s and eye-piece is thus practically trebled. The best-known prismatic telescopes are the Zeiss field-gla.s.ses.

[Ill.u.s.tration: FIG. 127.]

THE REFLECTING TELESCOPE.

We must not omit reference to the _reflecting_ telescope, so largely used by astronomers. The front end of the telescope is open, there being no object-gla.s.s. Rays from the object fall on a parabolic mirror situated in the rear end of the tube. This reflects them forwards to a focus. In the Newtonian reflector a plane mirror or prism is situated in the axis of the tube, at the focus, to reflect the rays through an eye-piece projecting through the side of the tube. Herschel's form of reflector has the mirror set at an angle to the axis, so that the rays are reflected direct into an eye-piece pointing through the side of the tube towards the mirror.

THE PARABOLIC MIRROR.

This mirror (Fig. 128) is of such a shape that all rays parallel to the axis are reflected to a common point. In the marine searchlight a powerful arc lamp is arranged with the arc at the focus of a parabolic reflector, which sends all reflected light forward in a pencil of parallel rays. The most powerful searchlight in existence gives a light equal to that of 350 million candles.

[Ill.u.s.tration: FIG. 128.--A parabolic reflector.]

THE COMPOUND MICROSCOPE.

We have already observed (Fig. 110) that the nearer an object approaches a lens the further off behind it is the real image formed, until the object has reached the focal distance, when no image at all is cast, as it is an infinite distance behind the lens. We will a.s.sume that a certain lens has a focus of six inches. We place a lighted candle four feet in front of it, and find that a _sharp_ diminished image is cast on a ground-gla.s.s screen held seven inches behind it. If we now exchange the positions of the candle and the screen, we shall get an enlarged image of the candle. This is a simple demonstration of the law of _conjugate foci_--namely, that the distance between the lens and an object on one side and that between the lens and the corresponding image on the other bear a definite relation to each other; and an object placed at either focus will cast an image at the other. Whether the image is larger or smaller than the object depends on which focus it occupies. In the case of the object-gla.s.s of a telescope the image was at what we may call the _short_ focus.

[Ill.u.s.tration: FIG. 129.--Diagram to explain the compound microscope.]

Now, a compound microscope is practically a telescope with the object at the _long_ focus, very close to a short-focus lens. A greatly enlarged image is thrown (see Fig. 129) at the conjugate focus, and this is caught and still further magnified by the eye-piece. We may add that the object-gla.s.s, or _objective_, of a microscope is usually compounded of several lenses, as is also the eye-piece.

THE MAGIC-LANTERN.

The most essential features of a magic-lantern are:--(1) The _source of light_; (2) the _condenser_ for concentrating the light rays on to the slide; (3) the _lens_ for projecting a magnified image on to a screen.

Fig. 130 shows these diagrammatically. The _illuminant_ is most commonly an oil-lamp, or an acetylene gas jet, or a cylinder of lime heated to intense luminosity by an oxy-hydrogen flame. The natural combustion of hydrogen is attended by a great heat, and when the supply of oxygen is artificially increased the temperature of the flame rises enormously.

The nozzle of an oxy-hydrogen jet has an interior pipe connected with the cylinder holding one gas, and an exterior, and somewhat larger, pipe leading from that containing the other, the two being arranged concentrically at the nozzle. By means of valves the proportions of the gases can be regulated to give the best results.

[Ill.u.s.tration: FIG. 130.--Sketch of the elements of a magic-lantern.]

The _condenser_ is set somewhat further from the illuminant than the princ.i.p.al focal length of the lenses, so that the rays falling on them are bent inwards, or to the slide.

The _objective_, or object lens, stands in front of the slide. Its position is adjustable by means of a rack and a draw-tube. The nearer it is brought to the slide the further away is the conjugate focus (see p.

239), and consequently the image. The exhibitor first sets up his screen and lantern, and then finds the conjugate foci of slide and image by racking the lens in or out.

If a very short focus objective be used, subjects of microscopic proportions can be projected on the screen enormously magnified. During the siege of Paris in 1870-71 the Parisians established a balloon and pigeon post to carry letters which had been copied in a minute size by photography. These copies could be enclosed in a quill and attached to a pigeon's wing. On receipt, the copies were placed in a special lantern and thrown as large writing on the screen. Micro-photography has since then made great strides, and is now widely used for scientific purposes, one of the most important being the study of the crystalline formations of metals under different conditions.

THE BIOSCOPE.

"Living pictures" are the most recent improvement in magic-lantern entertainments. The negatives from which the lantern films are printed are made by pa.s.sing a ribbon of sensitized celluloid through a special form of camera, which feeds the ribbon past the lens in a series of jerks, an exposure being made automatically by a revolving shutter during each rest. The positive film is placed in a lantern, and the intermittent movement is repeated; but now the source of illumination is behind the film, and light pa.s.ses outwards through the shutter to the screen. In the Urban bioscope the film travels at the rate of fifteen miles an hour, upwards of one hundred exposures being made every second.

The impression of continuous movement arises from the fact that the eye cannot get rid of a visual impression in less than one-tenth of a second. So that if a series of impressions follow one another more rapidly than the eye can rid itself of them the impressions will overlap, and give one of _motion_, if the position of some of the objects, or parts of the objects, varies slightly in each succeeding picture.[25]

THE PLANE MIRROR.

[Ill.u.s.tration: FIG. 131.]

This chapter may conclude with a glance at the common looking-gla.s.s. Why do we see a reflection in it? The answer is given graphically by Fig.

131. Two rays, A _b_, A _c_, from a point A strike the mirror M at the points _b_ and _c_. Lines _b_ N, _c_ O, drawn from these points perpendicular to the mirror are called their _normals_. The angles A _b_ N, A _c_ O are the _angles of incidence_ of rays A _b_, A _c_. The paths which the rays take after reflection must make angles with _b_ N and _c_ O respectively equal to A _b_ N, A _c_ O. These are the _angles of reflection_. If the eye is so situated that the rays enter it as in our ill.u.s.tration, an image of the point A is seen at the point A^1, in which the lines D _b_, E _c_ meet when produced backwards.

[Ill.u.s.tration: FIG. 132.]

When the vertical mirror is replaced by a horizontal reflecting surface, such as a pond (Fig. 132), the same thing happens. The point at which the ray from the reflection of the spire's tip to the eye appears to pa.s.s through the surface of the water must be so situated that if a line were drawn perpendicular to it from the surface the angles made by lines drawn from the real spire tip and from the observer's eye to the base of the perpendicular would be equal.

[22] Glazebrook, "Light," p. 157.

[23] Glazebrook, "Light," p. 157.

[24] Galileo was severely censured and imprisoned for daring to maintain that the earth moved round the sun, and revolved on its axis.

[25] For a full account of Animated Pictures the reader might advantageously consult "The Romance of Modern Invention," pp. 166 foll.

Chapter XIV.

SOUND AND MUSICAL INSTRUMENTS.

Nature of sound--The ear--Musical instruments--The vibration of strings--The sounding-board and the frame of a piano--The strings--The striking mechanism--The quality of a note.

Sound differs from light, heat, and electricity in that it can be propagated through matter only. Sound-waves are matter-waves, not ether-waves. This can be proved by placing an electric bell under the bell-gla.s.s of an air-pump and exhausting all the air. Ether still remains inside the gla.s.s, but if the bell be set in motion no sound is audible. Admit air, and the clang of the gong is heard quite plainly.

Sound resembles light and heat, however, thus far, that it can be concentrated by means of suitable lenses and curved surfaces. An _echo_ is a proof of its _reflection_ from a surface.

Before dealing with the various appliances used for producing sound-waves of a definite character, let us examine that wonderful natural apparatus