A Text-Book of Astronomy Part 10

Extreme limit of visible violet 3,900 Middle of the violet 4,060 " " blue 4,730 " " green 5,270 " " yellow 5,810 " " orange 5,970 " " red 7,000 Extreme limit of visible red 7,600

[Ill.u.s.tration: PLATE I. THE NORTHERN CONSTELLATIONS]

The phrase "extreme limit of visible violet" or red used above must be understood to mean that in general the eye is not able to detect radiant energy having a wave length less than 3,900 or greater than 7,600 tenth meters. Radiant energy, however, exists in waves of both greater and shorter length than the above, and may be readily detected by apparatus not subject to the limitations of the human eye--e. g., a common thermometer will show a rise of temperature when its bulb is exposed to radiant energy of wave length much greater than 7,600 tenth meters, and a photographic plate will be strongly affected by energy of shorter wave length than 3,900 tenth meters.

76. REFLECTION AND CONDENSATION OF WAVES.--When the waves produced by dropping a bullet into a bucket of water meet the sides of the bucket, they appear to rebound and are reflected back toward the center, and if the bullet is dropped very near the center of the bucket the reflected waves will meet simultaneously at this point and produce there by their combined action a wave higher than that which was reflected at the walls of the bucket. There has been a condensation of energy produced by the reflection, and this increased energy is shown by the greater amplitude of the wave. The student should not fail to notice that each portion of the wave has traveled out and back over the radius of the bucket, and that they meet simultaneously at the center because of this equality of the paths over which they travel, and the resulting equality of time required to go out and back. If the bullet were dropped at one side of the center, would the reflected waves produce _at any point_ a condensation of energy?

If the bucket were of elliptical instead of circular cross section and the bullet were dropped at one focus of the ellipse there would be produced a condensation of reflected energy at the other focus, since the sum of the paths traversed by each portion of the wave before and after reflection is equal to the sum of the paths traversed by every other portion, and all parts of the wave reach the second focus at the same time. Upon what geometrical principle does this depend?

The condensation of wave energy in the circular and elliptical buckets are special cases under the general principle that such a condensation will be produced at any point which is so placed that different parts of the wave front reach it simultaneously, whether by reflection or by some other means, as shown below.

The student will note that for the sake of greater precision we here say _wave front_ instead of wave. If in any wave we imagine a line drawn along the crest, so as to touch every drop which at that moment is exactly at the crest, we shall have what is called a wave front, and similarly a line drawn through the trough between two waves, or through any set of drops similarly placed on a wave, const.i.tutes a wave front.

77. MIRRORS AND LENSES.--That form of radiant energy which we recognize as light and heat may be reflected and condensed precisely as are the waves of water in the exercise considered above, but owing to the extreme shortness of the wave length in this case the reflecting surface should be very smooth and highly polished. A piece of gla.s.s hollowed out in the center by grinding, and with a light film of silver chemically deposited upon the hollow surface and carefully polished, is often used by astronomers for this purpose, and is called a concave mirror.

The radiant energy coming from a star or other distant object and falling upon the silvered face of such a mirror is reflected and condensed at a point a little in front of the mirror, and there forms an image of the star, which may be seen with the unaided eye, if it is held in the right place, or may be examined through a magnifying gla.s.s.

Similarly, an image of the sun, a planet, or a distant terrestrial object is formed by the mirror, which condenses at its appropriate place the radiant energy proceeding from each and every point in the surface of the object, and this, in common phrase, produces an image of the object.

Another device more frequently used by astronomers for the production of images (condensation of energy) is a lens which in its simplest form is a round piece of gla.s.s, thick in the center and thin at the edge, with a cross section, such as is shown at _A B_ in Fig. 38. If we suppose _E G D_ to represent a small part of a wave front coming from a very distant source of radiant energy, such as a star, this wave front will be practically a plane surface represented by the straight line _E D_, but in pa.s.sing through the lens this surface will become warped, since light travels slower in gla.s.s than in air, and the central part of the beam, _G_, in its onward motion will be r.e.t.a.r.ded by the thick center of the lens, more than _E_ or _D_ will be r.e.t.a.r.ded by the comparatively thin outer edges of _A B_. On the right of the lens the wave front therefore will be transformed into a curved surface whose exact character depends upon the shape of the lens and the kind of gla.s.s of which it is made. By properly choosing these the new wave front may be made a part of a sphere having its center at the point _F_ and the whole energy of the wave front, _E G D_, will then be condensed at _F_, because this point is equally distant from all parts of the warped wave front, and therefore is in a position to receive them simultaneously. The distance of _F_ from _A B_ is called the focal length of the lens, and _F_ itself is called the focus. The significance of this last word (Latin, _focus_ = fireplace) will become painfully apparent to the student if he will hold a common reading gla.s.s between his hand and the sun in such a way that the focus falls upon his hand.

[Ill.u.s.tration: FIG. 38.--Ill.u.s.trating the theory of lenses.]

All the energy transmitted by the lens in the direction _G F_ is concentrated upon a very small area at _F_, and an image of the object--e. g., a star, from which the light came--is formed here. Other stars situated near the one in question will also send beams of light along slightly different directions to the lens, and these will be concentrated, each in its appropriate place, in the _focal plane_, _F H_, pa.s.sed through the focus, _F_, perpendicular to the line, _F G_, and we shall find in this plane a picture of all the stars or other objects within the range of the lens.

[Ill.u.s.tration: FIG. 39.--Essential parts of a reflecting telescope.]

78. TELESCOPES.--The simplest kind of telescope consists of a concave mirror to produce images, and a magnifying gla.s.s, called an _eyepiece_, through which to examine them; but for convenience' sake, so that the observer may not stand in his own light, a small mirror is frequently added to this combination, as at _H_ in Fig. 39, where the lines represent the directions along which the energy is propagated. By reflection from this mirror the focal plane and the images are s.h.i.+fted to _F_, where they may be examined from one side through the magnifying gla.s.s _E_.

[Ill.u.s.tration: FIG. 40.--A simple form of refracting telescope.]

Such a combination of parts is called a _reflecting_ telescope, while one in which the images are produced by a lens or combination of lenses is called a _refracting_ telescope, the adjective having reference to the bending, refraction, produced by the gla.s.s upon the direction in which the energy is propagated. The customary arrangement of parts in such a telescope is shown in Fig. 40, where the part marked _O_ is called the objective and _V E_ (the magnifying gla.s.s) is the eyepiece, or ocular, as it is sometimes called.

Most objects with which we have to deal in using a telescope send to it not light of one color only, but a mixture of light of many colors, many different wave lengths, some of which are refracted more than others by the gla.s.s of which the lens is composed, and in consequence of these different amounts of refraction a single lens does not furnish a single image of a star, but gives a confused jumble of red and yellow and blue images much inferior in sharpness of outline (definition) to the images made by a good concave mirror. To remedy this defect it is customary to make the objective of two or more pieces of gla.s.s of different densities and ground to different shapes as is shown at _O_ in Fig. 40. The two pieces of gla.s.s thus mounted in one frame const.i.tute a compound lens having its own focal plane, shown at _F_ in the figure, and similarly the lenses composing the eyepiece have a focal plane between the eyepiece and the objective which must also fall at _F_, and in the use of a telescope the eyepiece must be pushed out or in until its focal plane coincides with that of the objective. This process, which is called focusing, is what is accomplished in the ordinary opera gla.s.s by turning a screw placed between the two tubes, and it must be carefully done with every telescope in order to obtain distinct vision.

79. MAGNIFYING POWER.--The amount by which a given telescope magnifies depends upon the focal length of the objective (or mirror) and the focal length of the eyepiece, and is equal to the ratio of these two quant.i.ties. Thus in Fig. 40 the distance of the objective from the focal plane _F_ is about 16 times as great as the distance of the eyepiece from the same plane, and the magnifying power of this telescope is therefore 16 diameters. A magnifying power of 16 diameters means that the diameter of any object seen in the telescope looks 16 times as large as it appears without the telescope, and is nearly equivalent to saying that the object appears only one sixteenth as far off. Sometimes the magnifying power is a.s.sumed to be the number of times that the _area_ of an object seems increased; and since areas are proportional to the squares of lines, the magnifying power of 16 diameters might be called a power of 256. Every large telescope is provided with several eyepieces of different focal lengths, ranging from a quarter of an inch to two and a half inches, which are used to furnish different magnifying powers as may be required for the different kinds of work undertaken with the instrument. Higher powers can be used with large telescopes than with small ones, but it is seldom advantageous to use with any telescope an eyepiece giving a higher power than 60 diameters for each inch of diameter of the objective.

The part played by the eyepiece in determining magnifying power will be readily understood from the following experiment:

Make a pin hole in a piece of cardboard. Bring a printed page so close to one eye that you can no longer see the letters distinctly, and then place the pin hole between the eye and the page. The letters which were before blurred may now be seen plainly through the pin hole, even when the page is brought nearer to the eye than before. As it is brought nearer, notice how the letters seem to become larger, solely because they are nearer. A pin hole is the simplest kind of a magnifier, and the eyepiece in a telescope plays the same part as does the pin hole in the experiment; it enables the eye to be brought nearer to the image, and the shorter the focal length of the eyepiece the nearer is the eye brought to the image and the higher is the magnifying power.

80. THE EQUATORIAL MOUNTING.--Telescopes are of all sizes, from the modest opera gla.s.s which may be carried in the pocket and which requires no other support than the hand, to the giant which must have a special roof to shelter it and elaborate machinery to support and direct it toward the sky. But for even the largest telescopes this machinery consists of the following parts, which are ill.u.s.trated, with exception of the last one, in the small equatorial telescope shown in Fig. 41. It is not customary to place a driving clock on so small a telescope as this:

(_a_) A supporting pier or tripod.

(_b_) An axis placed parallel to the axis of the earth.

(_c_) Another axis at right angles to _b_ and capable of revolving upon _b_ as an axle.

(_d_) The telescope tube attached to _c_ and capable of revolving about _c_.

(_e_) Graduated circles attached to _c_ and _b_ to measure the amount by which the telescope is turned on these axes.

(_f_) A driving clock so connected with _b_ as to make _c_ (and _d_) revolve about _b_ with an angular velocity equal and opposite to that with which the earth turns upon its axis.

[Ill.u.s.tration: FIG. 41.--A simple equatorial mounting.]

[Ill.u.s.tration: FIG. 42.--Equatorial mounting of the great telescope of the Yerkes Observatory.]

Such a support is called an equatorial mounting, and the student should note from the figure that the circles, _e_, measure the hour angle and declination of any star toward which the telescope is directed, and conversely if the telescope be so set that these circles indicate the hour angle and declination of any given star, the telescope will then point toward that star. In this way it is easy to find with the telescope any moderately bright star, even in broad daylight, although it is then absolutely invisible to the naked eye. The rotation of the earth about its axis will speedily carry the telescope away from the star, but if the driving clock be started, its effect is to turn the telescope toward the west just as fast as the earth's rotation carries it toward the east, and by these compensating motions to keep it directed toward the star. In Fig. 42, which represents the largest and one of the most perfect refracting telescopes ever built, let the student pick out and identify the several parts of the mounting above described. A part of the driving clock may be seen within the head of the pier. In Fig. 43 trace out the corresponding parts in the mounting of a reflecting telescope.

[Ill.u.s.tration: FIG. 43.--The reflecting telescope of the Paris Observatory.]

A telescope is often only a subordinate part of some instrument or apparatus, and then its style of mounting is determined by the requirements of the special case; but when the telescope is the chief thing, and the remainder of the apparatus is subordinate to it, the equatorial mounting is almost always adopted, although sometimes the arrangement of the parts is very different in appearance from any of those shown above. Beware of the popular error that an object held close in front of a telescope can be seen by an observer at the eyepiece. The numerous stories of astronomers who saw spiders crawling over the objective of their telescope, and imagined they were beholding strange objects in the sky, are all fict.i.tious, since nothing on or near the objective could possibly be seen through the telescope.

81. PHOTOGRAPHY.--A photographic camera consists of a lens and a device for holding at its focus a specially prepared plate or film. This plate carries a chemical deposit which is very sensitive to the action of light, and which may be made to preserve the imprint of any picture which the lens forms upon it. If such a sensitive plate is placed at the focus of a reflecting telescope, the combination becomes a camera available for astronomical photography, and at the present time the tendency is strong in nearly every branch of astronomical research to subst.i.tute the sensitive plate in place of the observer at a telescope.

A refracting telescope may also be used for astronomical photography, and is very much used, but some complications occur here on account of the resolution of the light into its const.i.tuent colors in pa.s.sing through the objective. Fig. 44 shows such a telescope, or rather two telescopes, one photographic, the other visual, supported side by side upon the same equatorial mounting.

[Ill.u.s.tration: FIG. 44.--Photographic telescope of the Paris Observatory.]

One of the great advantages of photography is found in connection with what is called--

82. PERSONAL EQUATION.--It is a remarkable fact, first investigated by the German astronomer Bessel, three quarters of a century ago, that where extreme accuracy is required the human senses can not be implicitly relied upon. The most skillful observers will not agree exactly in their measurement of an angle or in estimating the exact instant at which a star crossed the meridian; the most skillful artists can not draw identical pictures of the same object, etc.

These minor deceptions of the senses are included in the term _personal equation_, which is a famous phrase in astronomy, denoting that the observations of any given person require to be corrected by means of some equation involving his personality.

General health, digestion, nerves, fatigue, all influence the personal equation, and it was in reference to such matters that one of the most eminent of living astronomers has given this description of his habits of observing:

"In order to avoid every physiological disturbance, I have adopted the rule to abstain for one or two hours before commencing observations from every laborious occupation; never to go to the telescope with stomach loaded with food; to abstain from everything which could affect the nervous system, from narcotics and alcohol, and especially from the abuse of coffee, which I have found to be exceedingly prejudicial to the accuracy of observation."[3] A regimen suggestive of preparation for an athletic contest rather than for the more quiet labors of an astronomer.

[3] Schiaparelli, Osservazioni sulle Stelle Doppie.

83. VISUAL AND PHOTOGRAPHIC WORK.--The photographic plate has no stomach and no nerves, and is thus free from many of the sources of error which inhere in visual observations, and in special cla.s.ses of work it possesses other marked advantages, such as rapidity when many stars are to be dealt with simultaneously, permanence of record, and owing to the c.u.mulative effect of long exposure of the plate it is possible to photograph with a given telescope stars far too faint to be seen through it. On the other hand, the eye has the advantage in some respects, such as studying the minute details of a fairly bright object--e. g., the surface of a planet, or the sun's corona and, for the present at least, neither method of observing can exclude the other. For a remarkable case of discordance between the results of photographic and visual observations compare the pictures of the great nebula in the constellation Andromeda, which are given in Chapter XIV. A partial explanation of these discordances and other similar ones is that the eye is most strongly affected by greenish-yellow light, while the photographic plate responds most strongly to violet light; the photograph, therefore, represents things which the eye has little capacity for seeing, and _vice versa_.

84. THE SPECTROSCOPE.--In some respects the spectroscope is the exact counterpart of the telescope. The latter condenses radiant energy and the former disperses it. As a measuring instrument the telescope is mainly concerned with the direction from which light comes, and the different colors of which that light is composed affect it only as an obstacle to be overcome in its construction. On the other hand, with the spectroscope the direction from which the radiant energy comes is of minor consequence, and the all-important consideration is the intrinsic character of that radiation. What colors are present in the light and in what proportions? What can these colors be made to tell about the nature and condition of the body from which they come, be it sun, or star, or some terrestrial source of light, such as an arc lamp, a candle flame, or a furnace in blast? These are some of the characteristic questions of the spectrum a.n.a.lysis, and, as the name implies, they are solved by a.n.a.lyzing the radiant energy into its component parts, setting down the blue light in one place, the yellow in another, the red in still another, etc., and interpreting this array of colors by means of principles which we shall have to consider. Something of this process of color a.n.a.lysis may be seen in the brilliant hues shown by a soap bubble, or reflected from a piece of mother-of-pearl, and still more strikingly exhibited in the rainbow, produced by raindrops which break up the sunlight into its component colors and arrange them each in its appropriate place. Any of these natural methods of decomposing light might be employed in the construction of a spectroscope, but in spectroscopes which are used for a.n.a.lyzing the light from feeble sources, such as a star, or a candle flame, a gla.s.s prism of triangular cross section is usually employed to resolve the light into its component colors, which it does by refracting it as shown at the edges of the lens in Fig. 38.

[Ill.u.s.tration: FIG. 45.--Resolution of light into its component colors.]

The course of a beam of light in pa.s.sing through such a prism is shown in Fig. 45. Note that the bending of the light from its original course into a new one, which is here shown as produced by the prism, is quite similar to the bending shown at the edges of a lens and comes from the same cause, the slower velocity of light in gla.s.s than in air. It takes the light-waves as long to move over the path _A B_ in gla.s.s as over the longer path _1_, _2_, _3_, _4_, of which only the middle section lies in the gla.s.s.

Not only does the prism bend the beam of light transmitted by it, but it bends in different degree light of different colors, as is shown in the figure, where the beam at the left of the prism is supposed to be made up of a mixture of blue and red light, while at the right of the prism the greater deviation imparted to the blue quite separates the colors, so that they fall at different places on the screen, _S S_. The compound light has been a.n.a.lyzed into its const.i.tuents, and in the same way every other color would be put down at its appropriate place on the screen, and a beam of white light falling upon the prism would be resolved by it into a sequence of colors, falling upon the screen in the order red, orange, yellow, green, blue, indigo, violet. The initial letters of these names make the word _Roygbiv_, and by means of it their order is easily remembered.

[Ill.u.s.tration: FIG. 46.--Princ.i.p.al parts of a spectroscope.]

If the light which is to be examined comes from a star the a.n.a.lysis made by the prism is complete, and when viewed through a telescope the image of the star is seen to be drawn out into a band of light, which is called a _spectrum_, and is red at one end and violet or blue at the other, with all the colors of the rainbow intervening in proper order between these extremes. Such a prism placed in front of the objective of a telescope is called an objective prism, and has been used for stellar work with marked success at the Harvard College Observatory. But if the light to be a.n.a.lyzed comes from an object having an appreciable extent of surface, such as the sun or a planet, the objective prism can not be successfully employed, since each point of the surface will produce its own spectrum, and these will appear in the _view telescope_ superposed and confused one with another in a very objectionable manner. To avoid this difficulty there is placed between the prism and the source of light an opaque screen, _S_, with a very narrow slit cut in it, through which all the light to be a.n.a.lyzed must pa.s.s and must also go through a lens, _A_, placed between the slit and the prism, as shown in Fig. 46.

The slit and lens, together with the tube in which they are usually supported, are called a _collimator_. By this device a very limited amount of light is permitted to pa.s.s from the object through the slit and lens to the prism and is there resolved into a spectrum, which is in effect a series of images of the slit in light of different colors, placed side by side so close as to make practically a continuous ribbon of light whose width is the length of each individual picture of the slit. The length of the ribbon (dispersion) depends mainly upon the shape of the prism and the kind of gla.s.s of which it is made, and it may be very greatly increased and the efficiency of the spectroscope enhanced by putting two, three, or more prisms in place of the single one above described. When the amount of light is very great, as in the case of the sun or an electric arc lamp, it is advantageous to alter slightly the arrangement of the spectroscope and to subst.i.tute in place of the prism a grating--i. e., a metallic mirror with a great number of fine parallel lines ruled upon its surface at equal intervals, one from another. It is by virtue of such a system of fine parallel grooves that mother-of-pearl displays its beautiful color effects, and a brilliant spectrum of great purity and high dispersion is furnished by a grating ruled with from 10,000 to 20,000 lines to the inch. Fig. 47 represents, rather crudely, a part of the spectrum of an arc light furnished by such a grating, or rather it shows three different spectra arranged side by side, and looking something like a rude ladder. The sides of the ladder are the spectra furnished by the incandescent carbons of the lamp, and the cross pieces are the spectrum of the electric arc filling the s.p.a.ce between the carbons. Fig. 48 shows a continuation of the same spectra into a region where the radiant energy is invisible to the eye, but is capable of being photographed.

[Ill.u.s.tration: FIG. 47.--Green and blue part of the spectrum of an electric arc light.]

It is only when a lens is placed between the lamp and the slit of the spectroscope that the three spectra are shown distinct from each other as in the figure. The purpose of the lens is to make a picture of the lamp upon the slit, so that all the radiant energy from any one point of the arc may be brought to one part of the slit, and thus appear in the resulting spectrum separated from the energy which comes from every other part of the arc. Such an instrument is called an _a.n.a.lyzing spectroscope_ while one without the lens is called an _integrating spectroscope_, since it furnishes to each point of the slit a sample of the radiant energy coming from every part of the source of light, and thus produces only an average spectrum of that source without distinction of its parts. When a spectroscope is attached to a telescope, as is often done (see Fig. 49), the eyepiece is removed to make way for it, and the telescope objective takes the part of the a.n.a.lyzing lens. A camera is frequently combined with such an apparatus to photograph the spectra it furnishes, and the whole instrument is then called a _spectrograph_.