convex lens, or by a concave mirror. A convex lens is a flat piece of
glass, having its two faces convex, or spherical, as is seen in a common
sun-glass, or a pair of spectacles. Every one who has seen a sun-glass,
knows, that, when held towards the sun, it collects the solar rays into
a small bright circle in the focus. This is in fact a small _image_ of
the sun. In the same manner, the image of any distant object, as a star,
may be formed, as is represented in the following diagram. Let A B C D,
Fig. 4, represent the tube of the telescope. At the front end, or at the
end which is directed towards the object, (which we will suppose to be
the moon,) is inserted a convex lens, L, which receives the rays of
light from the moon, and collects them into the focus at _a_, forming an
image of the moon. This image is viewed by a magnifier attached to the
end B C. The lens, L, is called the _object-glass_, and the microscope
in B C, the _eyeglass_. We apply a microscope to this image just as we
would to any object; and, by greatly enlarging its dimensions, we may
render its various parts far more distinct than they would otherwise be;
while, at the same time, the lens collects and conveys to the eye a much
greater quantity of light than would proceed directly from the body
under examination. A very few rays of light only, from a distant object,
as a star, can enter the eye directly; but a lens one foot in diameter
will collect a beam of light of the same dimensions, and convey it to
the eye. By these means, many obscure celestial objects become
distinctly visible, which would otherwise be either too minute, or not
sufficiently luminous, to be seen by us.
But the image may also be formed by means of a _concave mirror_, which,
as well as the concave lens, has the property of collecting the rays of
light which proceed from any luminous body, and of forming an image of
that body. The image formed by a concave mirror is magnified by a
microscope, in the same manner as when formed by the concave lens. When
the lens is used to form an image, the instrument is called a
_refracting telescope_; when a concave mirror is used, it is called a
_reflecting telescope_.
The office of the object-glass is simply _to collect_ the light, and to
form an _image_ of the object, but not to magnify it: the magnifying
power is wholly in the eyeglass. Hence the principle of the telescope is
as follows: _By means of the object-glass_, (in the refracting
telescope,) _or by the concave mirror_, (in the reflecting telescope,)
_we form an image of the object_, _and magnify that image by a
microscope_.
The invention of this noble instrument is generally ascribed to the
great philosopher of Florence, Galileo. He had heard that a spectacle
maker of Holland had accidentally hit upon a discovery, by which distant
objects might be brought apparently nearer; and, without further
information, he pursued the inquiry, in order to ascertain what forms
and combinations of glasses would produce such a result. By a very
philosophical process of reasoning, he was led to the discovery of that
peculiar form of the telescope which bears his name.
Although the telescopes made by Galileo were no larger than a common
spyglass of the kind now used on board of ships, yet, as they gave new
views of the heavenly bodies, revealing the mountains and valleys of
the moon, the satellites of Jupiter, and multitudes of stars which are
invisible to the naked eye, it was regarded with infinite delight and
astonishment.
_Reflecting_ telescopes were first constructed by Sir Isaac Newton,
although the use of a concave reflector, instead of an object-glass, to
form the image, had been previously suggested by Gregory, an eminent
Scotch astronomer. The first telescope made by Newton was only six
inches long. Its reflector, too, was only a little more than an inch.
Notwithstanding its small dimensions, it performed so well, as to
encourage further efforts; and this illustrious philosopher afterwards
constructed much larger instruments, one of which, made with his own
hands, was presented to the Royal Society of London, and is now
carefully preserved in their library.
Newton was induced to undertake the construction of reflecting
telescopes, from the belief that refracting telescopes were necessarily
limited to a very small size, with only moderate illuminating powers,
whereas the dimensions and powers of the former admitted of being
indefinitely increased. Considerable _magnifying_ powers might, indeed,
be obtained from refractors, by making them very long; but the
_brightness_ with which telescopic objects are seen, depends greatly on
the dimensions of the beam of light which is collected by the
object-glass, or by the mirror, and conveyed to the eye; and therefore,
small object-glasses cannot have a very high illuminating power. Now,
the experiments of Newton on colors led him to believe, that it would be
impossible to employ large lenses in the construction of telescopes,
since such glasses would give to the images, they formed, the colors of
the rainbow. But later opticians have found means of correcting these
imperfections, so that we are now able to use object-glasses a foot or
more in diameter, which give very clear and bright images. Such
instruments are called _achromatic_ telescopes,--a name implying the
absence of prismatic or rainbow colors in the image. It is, however, far
more difficult to construct large achromatic than large reflecting
telescopes. Very large pieces of glass can seldom be found, that are
sufficiently pure for the purpose; since every inequality in the glass,
such as waves, tears, threads, and the like, spoils it for optical
purposes, as it distorts the light, and produces nothing but confused
images.
The achromatic telescope (that is, the refracting telescope, having such
an object-glass as to give a colorless image) was invented by Dollond, a
distinguished English artist, about the year 1757. He had in his
possession a quantity of glass of a remarkably fine quality, which
enabled him to carry his invention at once to a high degree of
perfection. It has ever since been, with the manufacturers of
telescopes, a matter of the greatest difficulty to find pieces of glass,
of a suitable quality for object-glasses, more than two or three inches
in diameter. Hence, large achromatic telescopes are very expensive,
being valued in proportion to the _cubes_ of their diameters; that is,
if a telescope whose aperture (as the breadth of the object-glass is
technically called) is two inches, cost one hundred dollars, one whose
aperture is eight inches would cost six thousand four hundred dollars.
Since it is so much easier to make large reflecting than large
refracting telescopes, you may ask, why the latter are ever attempted,
and why reflectors are not exclusively employed? I answer, that the
achromatic telescope, when large and well constructed, is a more perfect
and more durable instrument than the reflecting telescope. Much more of
the light that falls on the mirror is absorbed than is lost in passing
through the object-glass of a refractor; and hence the larger achromatic
telescopes afford a stronger light than the reflecting, unless the
latter are made of an enormous and unwieldy size. Moreover, the mirror
is very liable to tarnish, and will never retain its full lustre for
many years together; and it is no easy matter to restore the lustre,
when once impaired.
In my next Letter, I will give you an account of some of the most
celebrated telescopes that have ever been constructed, and point out the
method of using this excellent instrument, so as to obtain with it the
finest views of the heavenly bodies.
FOOTNOTE:
[2] From two Greek words, =têle=, (_tele_,) _far_, and =schopeô=,
(_skopeo_,) _to see_.
LETTER IV
TELESCOPE CONTINUED.
----"the broad circumference
Hung on his shoulders like the moon, whose orb
Through _optic glass_ the Tuscan artist views
At evening, from the top of Fesolé
Or in Valdarno, to descry new lands,
Rivers or mountains, in her spotted globe."--_Milton._
The two most celebrated telescopes, hitherto made, are Herschel's
_forty-feet reflector_, and the _great Dorpat refractor_. Herschel was a
Hanoverian by birth, but settled in England in the younger part of his
life. As early as 1774, he began to make telescopes for his own use;
and, during his life, he made more than four hundred, of various sizes
and powers. Under the patronage of George the Third, he completed, in
1789, his great telescope, having a tube of iron, forty feet long, and a
speculum, forty-nine and a half inches or more than four feet in
diameter. Let us endeavor to form a just conception of this gigantic
instrument, which we can do only by dwelling on its dimensions, and
comparing them with those of other objects with which we are familiar,
as the length or height of a house, and the breadth of a hogshead or
cistern, of known dimensions. The reflector alone weighed nearly a ton.
So large and ponderous an instrument must require a vast deal of
machinery to work it, and to keep it steady; and, accordingly, the
framework surrounding it was formed of heavy timbers, and resembled the
frame of a large building. When one of the largest of the fixed stars,
as Sirius, is entering the field of this telescope, its approach is
announced by a bright dawn, like that which precedes the rising sun; and
when the star itself enters the field, the light is insupportable to the
naked eye. The planets are expanded into brilliant luminaries, like the
moon; and innumerable multitudes of stars are scattered like glittering
dust over the celestial vault.
The great Dorpat telescope is of more recent construction. It was made
by Fraunhofer, a German optician of the greatest eminence, at Munich, in
Bavaria, and takes its name from its being attached to the observatory
at Dorpat, in Russia. It is of much smaller dimensions than the great
telescope of Herschel. Its object-glass is nine and a half inches in
diameter, and its length, fourteen feet. Although the price of this
instrument was nearly five thousand dollars, yet it is said that this
sum barely covered the actual expenses. It weighs five thousand pounds,
and yet is turned with the finger. In facility of management, it has
greatly the advantage of Herschel's telescope. Moreover, the sky of
England is so much of the time unfavorable for astronomical observation,
that _one hundred_ good hours (or those in which the higher powers can
be used) are all that can be obtained in a whole year. On this account,
and on account of the difficulty of shifting the position of the
instrument, Herschel estimated that it would take about six hundred
years to obtain with it even a momentary glimpse of every part of the
heavens. This remark shows that such great telescopes are unsuited to
the common purposes of astronomical observation. Indeed, most of
Herschel's discoveries were made with his small telescopes; and
although, for certain rare purposes, powers were applied which magnified
seven thousand times, yet, in most of his observations, powers
magnifying only two or three hundred times were employed. The highest
power of the Dorpat telescope is only seven hundred, and yet the
director of this instrument, Professor Struve, is of the opinion, that
it is nearly or quite equal in quality, all things considered, to
Herschel's forty-feet reflector.
It is not generally understood in what way greatness of size in a
telescope increases its powers; and it conveys but an imperfect idea of
the excellence of a telescope, to tell how much it magnifies. In the
same instrument, an increase of magnifying power is always attended with
a diminution of the light and of the field of view. Hence, the lower
powers generally afford the most agreeable views, because they give the
clearest light, and take in the largest space. The several circumstances
which influence the qualities of a telescope are, illuminating power,
distinctness, field of view, and magnifying power. Large mirrors and
large object-glasses are superior to smaller ones, because they collect
a larger beam of light, and transmit it to the eye. Stars which are
invisible to the naked eye are rendered visible by the telescope,
because this instrument collects and conveys to the eye a large beam of
the few rays which emanate from the stars; whereas a beam of these rays
of only the diameter of the pupil of the eye, would afford too little
light for distinct vision. In this particular, large telescopes have
great advantages over small ones. The great mirror of Herschel's
forty-feet reflector collects and conveys to the eye a beam more than
four feet in diameter. The Dorpat telescope also transmits to the eye a
beam nine and one half inches in diameter. This seems small, in
comparison with the reflector; but much less of the light is lost on
passing through the glass than is absorbed by the mirror, and the mirror
is very liable to be clouded or tarnished; so that there is not so great
a difference in the two instruments, in regard to illuminating power, as
might be supposed from the difference of size.
_Distinctness of view_ is all-important to the performance of an
instrument. The object may be sufficiently bright, yet, if the image is
distorted, or ill-defined, the illumination is of little consequence.
This property depends mainly on the skill with which all the
imperfections of figure and color in the glass or mirror are corrected,
and can exist in perfection only when the image is rendered completely
achromatic, and when all the rays that proceed from each point in the
object are collected into corresponding points of the image,
unaccompanied by any other rays. Distinctness is very much affected by
the _steadiness_ of the instrument. Every one knows how indistinct a
page becomes, when a book is passed rapidly backwards and forwards
before the eyes, and how difficult it is to read in a carriage in rapid
motion on a rough road.
_Field of view_ is another important consideration. The finest
instruments exhibit the moon, for example, not only bright and distinct,
in all its parts, but they take in the whole disk at once; whereas, the
inferior instruments, when the higher powers, especially, are applied,
permit us to see only a small part of the moon at once.
I hope, my friend, that, when you have perused these Letters, or rather,
while you are perusing them, you will have frequent opportunities of
looking through a good telescope. I even anticipate that you will
acquire such a taste for viewing the heavenly bodies with the aid of a
good glass, that you will deem a telescope a most suitable appendage to
your library, and as certainly not less an ornament to it than the more
expensive statues with which some people of fortune adorn theirs. I will
therefore, before concluding this letter, offer you a few _directions
for using the telescope_.
Some states of weather, even when the sky is clear, are far more
favorable for astronomical observation than others. After sudden changes
of temperature in the atmosphere, the medium is usually very unsteady.
If the sun shines out warm after a cloudy season, the ground first
becomes heated, and the air that is nearest to it is expanded, and
rises, while the colder air descends, and thus ascending and descending
currents of air, mingling together, create a confused and wavy medium.
The same cause operates when a current of hot air rises from a chimney;
and hence the state of the atmosphere in cities and large towns is very
unfavorable to the astronomer, on this account, as well as on account
of the smoky condition in which it is usually found. After a long season
of dry weather, also, the air becomes smoky, and unfit for observation.
Indeed, foggy, misty, or smoky, air is so prevalent in some countries,
that only a very few times in the whole year can be found, which are
entirely suited to observation, especially with the higher powers; for
we must recollect, that these inequalities and imperfections are
magnified by telescopes, as well as the objects themselves. Thus, as I
have already mentioned, not more than one hundred good hours in a year
could be obtained for observation with Herschel's great telescope. By
_good_ hours, Herschel means that the sky must be very clear, the moon
absent, no twilight, no haziness, no violent wind, and no sudden change
of temperature. As a general fact, the warmer climates enjoy a much
finer sky for the astronomer than the colder, having many more clear
evenings, a short twilight, and less change of temperature. The watery
vapor of the atmosphere, also, is more perfectly dissolved in hot than
in cold air, and the more water air contains, provided it is in a state
of perfect solution, the clearer it is.
A _certain preparation of the observer himself_ is also requisite for
the nicest observations with the telescope. He must be free from all
agitation, and the eye must not recently have been exposed to a strong
light, which contracts the pupil of the eye. Indeed, for delicate
observations, the observer should remain for some time beforehand in a
dark room, to let the pupil of the eye dilate. By this means, it will be
enabled to admit a larger number of the rays of light. In ascending the
stairs of an observatory, visitors frequently get out of breath, and
having perhaps recently emerged from a strongly-lighted apartment, the
eye is not in a favorable state for observation. Under these
disadvantages, they take a hasty look into the telescope, and it is no
wonder that disappointment usually follows.
Want of steadiness is a great difficulty attending the use of the
highest magnifiers; for the motions of the instrument are magnified as
well as the object. Hence, in the structure of observatories, the
greatest pains is requisite, to avoid all tremor, and to give to the
instruments all possible steadiness; and the same care is to be
exercised by observers. In the more refined observations, only one or
two persons ought to be near the instrument.
In general, _low powers_ afford better views of the heavenly bodies than
very high magnifiers. It may be thought absurd, to recommend the use of
low powers, in respect to large instruments especially, since it is
commonly supposed that the advantage of large instruments is, that they
will bear high magnifying powers. But this is not their only, nor even
their principal, advantage. A good light and large field are qualities,
for most purposes, more important than great magnifying power; and it
must be borne in mind, that, as we increase the magnifying power in a
given instrument, we diminish both the illumination and the field of
view. Still, different objects require different magnifying powers; and
a telescope is usually furnished with several varieties of powers, one
of which is best fitted for viewing the moon, another for Jupiter, and a
still higher power for Saturn. Comets require only the lowest
magnifiers; for here, our object is to command as much light, and as
large a field, as possible, while it avails little to increase the
dimensions of the object. On the other hand, for certain double stars,
(stars which appear single to the naked eye, but double to the
telescope,) we require very high magnifiers, in order to separate these
minute objects so far from each other, that the interval can be
distinctly seen. Whenever we exhibit celestial objects to inexperienced
observers, it is useful to precede the view with good _drawings_ of the
objects, accompanied by an explanation of what each appearance,
exhibited in the telescope, indicates. The novice is told, that
mountains and valleys can be seen in the moon by the aid of the
telescope; but, on looking, he sees a confused mass of light and shade,
and nothing which looks to him like either mountains or valleys. Had his
attention been previously directed to a plain drawing of the moon, and
each particular appearance interpreted to him, he would then have looked
through the telescope with intelligence and satisfaction.
LETTER V.
OBSERVATORIES.
"We, though from heaven remote, to heaven will move,
With strength of mind, and tread the abyss above;
And penetrate, with an interior light,
Those upper depths which Nature hid from sight.
Pleased we will be, to walk along the sphere
Of shining stars, and travel with the year."--_Ovid._
An observatory is a structure fitted up expressly for astronomical
observations, and furnished with suitable instruments for that purpose.
The two most celebrated observatories, hitherto built, are that of Tycho
Brahe, and that of Greenwich, near London. The observatory of Tycho
Brahe, Fig. 5, was constructed at the expense of the King of Denmark, in
a style of royal magnificence, and cost no less than two hundred
thousand crowns. It was situated on the island of Huenna, at the
entrance of the Baltic, and was called Uraniburg, or the palace of the
skies.
Before I give you an account of Tycho's observatory, I will recite a few
particulars respecting this great astronomer himself.
Tycho Brahe was of Swedish descent, and of noble family; but having
received his education at the University of Copenhagen, and spent a
large part of his life in Denmark, he is usually considered as a Dane,
and quoted as a Danish astronomer. He was born in the year 1546. When he
was about fourteen years old, there happened a great eclipse of the sun,
which awakened in him a high interest, especially when he saw how
[Illustration Fig. 5.] accurately all the circumstances of it answered
to the prediction with which he had been before made acquainted. He was
immediately seized with an irresistible passion to acquire a knowledge
of the science which could so successfully lift the veil of futurity.
His friends had destined him for the profession of law, and, from the
superior talents of which he gave early promise, and with the advantage
of powerful family connexions, they had marked out for him a
distinguished career in public life. They therefore endeavored to
discourage him from pursuing a path which they deemed so much less
glorious than that, and vainly sought, by various means, to extinguish
the zeal for astronomy which was kindled in his youthful bosom.
Despising all the attractions of a court, he contracted an alliance with
a peasant girl, and, in the peaceful retirement of domestic life,
desired no happier lot than to peruse the grand volume which the
nocturnal heavens displayed to his enthusiastic imagination. He soon
established his fame as one of the greatest astronomers of the age, and
monarchs did homage to his genius. The King of Denmark became his
munificent patron, and James the First, King of England, when he went to
Denmark to complete his marriage with a Danish Princess, passed eight
days with Tycho in his observatory, and, at his departure, addressed to
the astronomer a Latin ode, accompanied with a magnificent present. He
gave him also his royal license to print his works in England, and added
to it the following complimentary letter: "Nor am I acquainted with
these things on the relation of others, or from a mere perusal of your
works, but I have seen them with my own eyes, and heard them with my own
ears, in your residence at Uraniburg, during the various learned and
agreeable conversations which I there held with you, which even now
affect my mind to such a degree, that it is difficult to decide, whether
I recollect them with greater pleasure or admiration." Admiring
disciples also crowded to this sanctuary of the sciences, to acquire a
knowledge of the heavens.