INSTRUMENTS AND THE PRINCIPLES INVOLVED IN THEIR USE
74. TWO FAMILIAR INSTRUMENTS.--In previous chapters we have seen that a
clock and a divided circle (protractor) are needed for the observations
which an astronomer makes, and it is worth while to note here that the
geography of the sky and the science of celestial motions depend
fundamentally upon these two instruments. The protractor is a simple
instrument, a humble member of the family of divided circles, but untold
labor and ingenuity have been expended on this family to make possible
the construction of a circle so accurately divided that with it angles
may be measured to the tenth of a second instead of to the tenth of a
degree--i. e., 3,600 times as accurate as the protractor furnishes.
The building of a good clock is equally important and has cost a like
amount of labor and pains, so that it is a far cry from Galileo and his
discovery that a pendulum "keeps time" to the modern clock with its
accurate construction and elaborate provision against disturbing
influences of every kind. Every such timepiece, whether it be of the
nutmeg variety which sells for a dollar, or whether it be the standard
clock of a great national observatory, is made up of the same essential
parts that fall naturally into four classes, which we may compare with
the departments of a well-ordered factory: I. A timekeeping department,
the pendulum or balance spring, whose oscillations must all be of equal
duration. II. A power department, the weights or mainspring, which,
when wound, store up the power applied from outside and give it out
piecemeal as required to keep the first department running. III. A
publication department, the dial and hands, which give out the time
furnished by Department I. IV. A transportation department, the wheels,
which connect the other three and serve as a means of transmitting power
and time from one to the other. The case of either clock or watch is
merely the roof which shelters it and forms no department of its
industry. Of these departments the first is by far the most important,
and its good or bad performance makes or mars the credit of the clock.
Beware of meddling with the balance wheel of your watch.
75. RADIANT ENERGY.--But we have now to consider other instruments which
in practice supplement or displace the simple apparatus hitherto
employed. Among the most important of these modern instruments are the
telescope, the spectroscope, and the photographic camera; and since all
these instruments deal with the light which comes from the stars to the
earth, we must for their proper understanding take account of the nature
of that light, or, more strictly speaking, we must take account of the
radiant energy emitted by the sun and stars, which energy, coming from
the sun, is translated by our nerves into the two different sensations
of light and heat. The radiant energy which comes from the stars is not
fundamentally different from that of the sun, but the amount of energy
furnished by any star is so small that it is unable to produce through
our nerves any sensible perception of heat, and for the same reason the
vast majority of stars are invisible to the unaided eye; they do not
furnish a sufficient amount of energy to affect the optic nerves. A hot
brick taken into the hand reveals its presence by the two different
sensations of heat and pressure (weight); but as there is only one brick
to produce the two sensations, so there is only one energy to produce
through its action upon different nerves the two sensations of light
and heat, and this energy is called _radiant_ because it appears to
stream forth radially from everything which has the capacity of emitting
it. For the detailed study of radiant energy the student is referred to
that branch of science called physics; but some of its elementary
principles may be learned through the following simple experiment, which
the student should not fail to perform for himself:
Drop a bullet or other similar object into a bucket of water and observe
the circular waves which spread from the place where it enters the
water. These waves are a form of radiant energy, but differing from
light or heat in that they are visibly confined to a single plane, the
surface of the water, instead of filling the entire surrounding space.
By varying the size of the bucket, the depth of the water, the weight of
the bullet, etc., different kinds of waves, big and little, may be
produced; but every such set of waves may be described and defined in
all its principal characteristics by means of three numbers--viz., the
vertical height of the waves from hollow to crest; the distance of one
wave from the next; and the velocity with which the waves travel across
the water. The last of these quantities is called the velocity of
propagation; the second is called the wave length; one half of the first
is called the amplitude; and all these terms find important applications
in the theory of light and heat.
The energy of the falling bullet, the disturbance which it produced on
entering the water, was carried by the waves from the center to the edge
of the bucket but not beyond, for the wave can go only so far as the
water extends. The transfer of energy in this way requires a perfectly
continuous medium through which the waves may travel, and the whole
visible universe is supposed to be filled with something called _ether_,
which serves everywhere as a medium for the transmission of radiant
energy just as the water in the experiment served as a medium for
transmitting in waves the energy furnished to it by the falling bullet.
The student may think of this energy as being transmitted in spherical
waves through the ether, every glowing body, such as a star, a candle
flame, an arc lamp, a hot coal, etc., being the origin and center of
such systems of waves, and determining by its own physical and chemical
properties the wave length and amplitude of the wave systems given off.
The intensity of any light depends upon the amplitude of the
corresponding vibration, and its color depends upon the wave length. By
ingenious devices which need not be here described it has been found
possible to measure the wave length corresponding to different
colors--e. g., all of the colors of the rainbow, and some of these wave
lengths expressed in tenth meters are as follows: A tenth meter is the
length obtained by dividing a meter into 10^{10} equal parts. 10^{10} =
10,000,000,000.
Color. Wave length.
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
[Illustration: 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, constitutes 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 glass 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 glass.
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 glass, 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 passing through the lens this surface will become warped, since light
travels slower in glass than in air, and the central part of the beam,
_G_, in its onward motion will be retarded by the thick center of the
lens, more than _E_ or _D_ will be retarded 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 glass 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 glass between his hand and the sun in such a way
that the focus falls upon his hand.
[Illustration: FIG. 38.--Illustrating 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_, passed 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.
[Illustration: 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 glass, 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 shifted
to _F_, where they may be examined from one side through the magnifying
glass _E_.
[Illustration: 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 glass 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 glass) 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 glass 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 glass of
different densities and ground to different shapes as is shown at _O_ in
Fig. 40. The two pieces of glass thus mounted in one frame constitute 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
glass 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
quantities. 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 assumed 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 glass 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 illustrated, 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.
[Illustration: FIG. 41.--A simple equatorial mounting.]
[Illustration: 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.
[Illustration: 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 fictitious, 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
substitute 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 constituent colors in passing
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.
[Illustration: 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 classes 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
cumulative 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 analysis, and, as the name implies, they are solved by
analyzing 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 analysis 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 analyzing the light from feeble
sources, such as a star, or a candle flame, a glass 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.
[Illustration: FIG. 45.--Resolution of light into its component colors.]
The course of a beam of light in passing 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 glass than in air. It takes
the light-waves as long to move over the path _A B_ in glass as over the
longer path _1_, _2_, _3_, _4_, of which only the middle section lies in
the glass.
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 analyzed into its constituents, 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.
[Illustration: FIG. 46.--Principal parts of a spectroscope.]
If the light which is to be examined comes from a star the analysis 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 analyzed 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 analyzed must pass 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 pass 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 glass 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 substitute 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 space
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.
[Illustration: 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 _analyzing
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
analyzing 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_.
[Illustration: FIG. 48.--Violet and ultraviolet parts of spectrum of an
arc lamp.]
[Illustration: FIG. 49.--A spectroscope attached to the Yerkes
telescope.]
85. SPECTRUM ANALYSIS.--Having seen the mechanism of the spectroscope by
which the light incident upon it is resolved into its constituent parts
and drawn out into a series of colors arranged in the order of their
wave lengths, we have now to consider the interpretation which is to be
placed upon the various kinds of spectra which may be seen, and here we
rely upon the experience of physicists and chemists, from whom we learn
as follows:
The radiant energy which is analyzed by the spectroscope has its source
in the atoms and molecules which make up the luminous body from which
the energy is radiated, and these atoms and molecules are able to
impress upon the ether their own peculiarities in the shape of waves of
different length and amplitude. We have seen that by varying the
conditions of the experiment different kinds of waves may be produced in
a bucket of water; and as a study of these waves might furnish an index
to the conditions which produced them, so the study of the waves
peculiar to the light which comes from any source may be made to give
information about the molecules which make up that source. Thus the
molecules of iron produce a system of waves peculiar to themselves and
which can be duplicated by nothing else, and every other substance gives
off its own peculiar type of energy, presenting a limited and definite
number of wave lengths dependent upon the nature and condition of its
molecules. If these molecules are free to behave in their own
characteristic fashion without disturbance or crowding, they emit light
of these wave lengths only, and we find in the spectrum a series of
bright lines, pictures of the slit produced by light of these particular
wave lengths, while between these bright lines lie dark spaces showing
the absence from the radiant energy of light of intermediate wave
lengths. Such a spectrum is shown in the central portion of Fig. 47,
which, as we have already seen, is produced by the space between the
carbons of the arc lamp. On the other hand, if the molecules are closely
packed together under pressure they so interfere with each other as to
give off a jumble of energy of all wave lengths, and this is translated
by the spectroscope into a continuous ribbon of light with no dark
spaces intervening, as in the upper and lower parts of Figs. 47 and 48,
produced by the incandescent solid carbons of the lamp. These two types
are known as the continuous and discontinuous spectrum, and we may lay
down the following principle regarding them:
A discontinuous spectrum, or bright-line spectrum as it is familiarly
called, indicates that the molecules of the source of light are not
crowded together, and therefore the light must come from an incandescent
gas. A continuous spectrum shows only that the molecules are crowded
together, or are so numerous that the body to which they belong is not
transparent and gives no further information. The body may be solid,
liquid, or gaseous, but in the latter case the gas must be under
considerable pressure or of great extent.
A second principle is: The lines which appear in a spectrum are
characteristic of the source from which the light came--e. g., the
double line in the yellow part of the spectrum at the extreme left in
Fig. 47 is produced by sodium vapor in and around the electric arc and
is never produced by anything but sodium. When by laboratory experiments
we have learned the particular set of lines corresponding to iron, we
may treat the presence of these lines in another spectrum as proof that
iron is present in the source from which the light came, whether that
source be a white-hot poker in the next room or a star immeasurably
distant. The evidence that iron is present lies in the nature of the
light, and there is no reason to suppose that nature to be altered on
the way from star to earth. It may, however, be altered by something
happening to the source from which it comes--e. g., changing temperature
or pressure may affect, and does affect, the spectrum which such a
substance as iron emits, and we must be prepared to find the same
substance presenting different spectra under different conditions, only
these conditions must be greatly altered in order to produce radical
changes in the spectrum.
[Illustration: FIG. 50.--The chief lines in the spectrum of
sunlight.--HERSCHEL.]
86. WAVE LENGTHS.--To identify a line as belonging to and produced by
iron or any other substance, its position in the spectrum--i. e., its
wave length--must be very accurately determined, and for the
identification of a substance by means of its spectrum it is often
necessary to determine accurately the wave lengths of many lines. A
complicated spectrum may consist of hundreds or thousands of lines, due
to the presence of many different substances in the source of light, and
unless great care is taken in assigning the exact position of these
lines in the spectrum, confusion and wrong identifications are sure to
result. For the measurement of the required wave length a tenth meter
(§ 75) is the unit employed, and a scale of wave lengths expressed in
this unit is presented in Fig. 50. The accuracy with which some of these
wave lengths are determined is truly astounding; a ten-billionth of an
inch! These numerical wave lengths save all necessity for referring to
the color of any part of the spectrum, and pictures of spectra for
scientific use are not usually printed in colors.
87. ABSORPTION SPECTRA.--There is another kind of spectrum, of greater
importance than either of those above considered, which is well
illustrated by the spectrum of sunlight (Fig. 50). This is a nearly
continuous spectrum crossed by numerous _dark_ lines due to absorption
of radiant energy in a comparatively cool gas through which it passes on
its way to the spectroscope. Fraunhofer, who made the first careful
study of spectra, designated some of the more conspicuous of these lines
by letters of the alphabet which are shown in the plate, and which are
still in common use as names for the lines, not only in the spectrum of
sunlight but wherever they occur in other spectra. Thus the double line
marked _D_, wave length 5893, falls at precisely the same place in the
spectrum as does the double (sodium) line which we have already seen in
the yellow part of the arc-light spectrum, which line is also called _D_
and bears a very intimate relation to the dark _D_ line of the solar
spectrum.
The student who has access to colored crayons should color one edge of
Fig. 50 in accordance with the lettering there given and, so far as
possible, he should make the transition from one color to the next a
gradual one, as it is in the rainbow.
Fig. 50 is far from being a complete representation of the spectrum of
sunlight. Not only does this spectrum extend both to the right and to
the left into regions invisible to the human eye, but within the limits
of the figure, instead of the seventy-five lines there shown, there are
literally thousands upon thousands of lines, of which only the most
conspicuous can be shown in such a cut as this.
The dark lines which appear in the spectrum of sunlight can, under
proper conditions, be made to appear in the spectrum of an arc light,
and Fig. 51 shows a magnified representation of a small part of such a
spectrum adjacent to the _D_ (sodium) lines. Down the middle of each of
these lines runs a black streak whose position (wave length) is
precisely that of the _D_ lines in the spectrum of sunlight, and whose
presence is explained as follows:
The very hot sodium vapor at the center of the arc gives off its
characteristic light, which, shining through the outer and cooler layers
of sodium vapor, is partially absorbed by these, resulting in a fine
dark line corresponding exactly in position and wave length to the
bright lines, and seen against these as a background, since the higher
temperature at the center of the arc tends to broaden the bright lines
and make them diffuse. Similarly the dark lines in the spectrum of the
sun (Fig. 50) point to the existence of a surrounding envelope of
relatively cool gases, which absorb from the sunlight precisely those
kinds of radiant energy which they would themselves emit if
incandescent. The resulting dark lines in the spectrum are to be
interpreted by the same set of principles which we have above applied to
the bright lines of a discontinuous spectrum, and they may be used to
determine the chemical composition of the sun, just as the bright lines
serve to determine the chemical elements present in the electric arc.
With reference to the mode of their formation, bright-line and dark-line
spectra are sometimes called respectively _emission_ and _absorption_
spectra.
[Illustration: FIG. 51.--The lines reversed.]
88. TYPES OF SPECTRUM.--The sun presents by far the most complex
spectrum known, and Fig. 50 shows only a small number of the more
conspicuous lines which appear in it. Spectra of stars, _per contra_,
appear relatively simple, since their feeble light is insufficient to
bring out faint details. In Chapters XIII and XIV there are shown types
of the different kinds of spectra given by starlight, and these are to
be interpreted by the principles above established. Thus the spectrum of
the bright star [b] Aurigæ shows a continuous spectrum crossed by a few
heavy absorption lines which are known from laboratory experiments to be
produced only by hydrogen. There must therefore be an atmosphere of
relatively cool hydrogen surrounding this star. The spectrum of Pollux
is quite similar to that of the sun and is to be interpreted as showing
a physical condition similar to that of the sun, while the spectrum of
[a] Herculis is quite different from either of the others. In subsequent
chapters we shall have occasion to consider more fully these different
types of spectrum.
89. THE DOPPLER PRINCIPLE.--This important principle of the spectrum
analysis is most readily appreciated through the following experiment:
Listen to the whistle of a locomotive rapidly approaching, and observe
how the pitch changes and the note becomes more grave as the locomotive
passes by and commences to recede. During the approach of the whistle
each successive sound wave has a shorter distance to travel in coming to
the ear of the listener than had its predecessor, and in consequence the
waves appear to come in quicker succession, producing a higher note with
a correspondingly shorter wave length than would be heard if the same
whistle were blown with the locomotive at rest. On the other hand, the
wave length is increased and the pitch of the note lowered by the
receding motion of the whistle. A similar effect is produced upon the
wave length of light by a rapid change of distance between the source
from which it comes and the instrument which receives it, so that a
diminishing distance diminishes very slightly the wave length of every
line in the spectrum produced by the light, and an increasing distance
increases these wave lengths, and this holds true whether the change of
distance is produced by motion of the source of light or by motion of
the instrument which receives it.
This change of wave length is sometimes described by saying that when a
body is rapidly approaching, the lines of its spectrum are all displaced
toward the violet end of the spectrum, and are correspondingly displaced
toward the red end by a receding motion. The amount of this shifting,
when it can be measured, measures the velocity of the body along the
line of sight, but the observations are exceedingly delicate, and it is
only in recent years that it has been found possible to make them with
precision. For this purpose there is made to pass through the
spectroscope light from an artificial source which contains one or more
chemical elements known to be present in the star which is to be
observed, and the corresponding lines in the spectrum of this light and
in the spectrum of the star are examined to determine whether they
exactly match in position, or show, as they sometimes do, a slight
displacement, as if one spectrum had been slipped past the other. The
difficulty of the observations lies in the extremely small amount of
this slipping, which rarely if ever in the case of a moving star amounts
to one sixth part of the interval between the close parallel lines
marked _D_ in Fig. 50. The spectral lines furnished by the headlight of
a locomotive running at the rate of a hundred miles per hour would be
displaced by this motion less than one six-thousandth part of the space
between the _D_ lines, an amount absolutely imperceptible in the most
powerful spectroscope yet constructed. But many of the celestial bodies
have velocities so much greater than a hundred miles per hour that these
may be detected and measured by means of the Doppler principle.
90. OTHER INSTRUMENTS.--Other instruments of importance to the
astronomer, but of which only casual mention can here be made, are the
meridian-circle; the transit, one form of which is shown in Fig. 52, and
the zenith telescope, which furnish refined methods for making
observations similar in kind to those which the student has already
learned to make with plumb line and protractor; the sextant, which is
pre-eminently the sailor's instrument for finding the latitude and
longitude at sea, by measuring the altitudes of sun and stars above the
sea horizon; the heliometer, which serves for the very accurate
measurement of small angles, such as the angular distance between two
stars not more than one or two degrees apart; and the photometer, which
is used for measuring the amount of light received from the celestial
bodies.
[Illustration: FIG. 52.--A combined transit instrument and zenith
telescope.]