of Nova Persei, but of all new stars, is a modification of the
foregoing. This theory is that the new star in its flight through
space suddenly plunged into a nebula, or into some portion of
space denser than that through which it had already passed. This
explanation is not only intelligible but reasonable. If the new star
plunged into a region filled with matter even as rare as air, the
friction would immediately set the star on fire. We see the same
phenomenon every night when a meteor hustles through our atmosphere.
The meteoric rocks, with the chill of empty space in and around them,
dash into our upper air. A few seconds are ample for the practical
annihilation of most of them: in that brief space of time they have
been subjected to a heat many times greater than that of a Bessemer
furnace.
We can imagine Nova Persei as some monster meteor, a meteor larger
than the sun, plunging into a gaseous mass somewhat like our air. In
a few hours its temperature would be increased a million-fold. This
increase would fill the surrounding space with fire, and there would
be an immense and ever-increasing area at fervent heat.
To the mind of the writer this explanation has most to commend it.
It is the one that is most in harmony with the information which has
been gathered by hundreds of observers aided by the finest of modern
scientific equipment. But there are other explanations. There will
always be other explanations so long as the world lasts.
One of these explanations is of more interest than the rest, inasmuch
as it makes a link of connection between the recent terrible volcanic
eruption in the West Indies and the sudden appearance of a new star
like Nova Persei. It is suggested that Nova Persei is, or rather was,
a world somewhat like our own, only vastly larger—that is, there was
an inner core of molten matter and an outer shell of solid material.
One day, according to the explosion theory, this outer shell burst,
and the interior fires rushed hither and thither like a devouring
flood all over the stellar globe. Vast chemical changes went on as
the lambent flames turned everything solid into streams of lava.
Great electrical disturbances took place all round the star. The
whole phenomenon of Nova Persei, according to this theory, is just
the destruction of St. Pierre on a sidereal scale.
Such a doom, of course, is possible in any star or planet whose
interior is still molten. At any moment the imprisoned fires might
break their barriers and change a cold, fruitful, life-bearing earth
into a furnace; but it is far from probable that any such fate will
ever be meted out to our planet or to any other, and, at any rate,
destruction did not come to Nova Persei in this manner. No explosion
could account for an access of heat and light any way comparable to
that which was observed. Neither could any interior disruption be
violent enough to hurl the star into fragments. The gravitational
hold of the star would prevent this dismemberment. Yet during the
ages the mind of man has been irresistibly drawn to this conception
of the world’s end, so much so that perhaps, after all, our instinct
is right and our science wrong, and the vision of the Minorite Celano
of the
Dies iræ, dies illa
Solvet sæculum in favilla,
is a vision of those things that will be in the later days.
We have already touched on one strange circumstance connected
with the appearance of Nova Persei. Dr. Anderson saw it for the
first time at a few minutes to three o’clock on the morning of 22d
February—that is, the news of the strange occurrence reached our
planet then; but when did the event actually take place?
At Greenwich and at some of the other foremost observatories attempts
have been made directly and indirectly to determine the distance of
Nova Persei. And yet this distance defies measurement. The star is
so far away that we have no instruments refined enough to deal with
the problem. But we know that the sudden blazing up of Nova Persei
was over and done with before our great-grandfathers were born.
It happened more than two hundred years ago—perhaps two thousand
years ago. All this time the news was swiftly traveling earthward,
traveling on and on and on, two hundred thousand miles every second
of the clock, past star and nebula and system, never halting, never
faltering—yet it took hundreds of years to come to us; and beyond us
lie countless worlds that will not see the new star for centuries to
come. Hundreds of years hence in _their_ sky will appear suddenly
in the constellation of Perseus a strange star; it will increase in
brightness for a few days just as it did in ours; it will fade away
intermittently just as it did in ours. There is no imagination here;
only sober facts.
We may be allowed, in closing our narrative of this wonderful star,
to make one excursion into the region of imagination. As the news
of the star passes on through space, are there any beings beyond
ourselves who will take record of its appearance? It has taken
centuries to come to us. Did any other creatures in some far-off
world lift their eyes to the stars and wonder, as we do, what all
this meant? Will some mortal, like ourselves, in some remoter world,
in a day yet to come, see the sight, and have the intelligence
to say, “Lo! a new star?” We have room enough here for the most
extravagant fancy. Perhaps there is so much room that we shall lose
ourselves if we venture to stray in such directions.
TELESCOPES.—A. FOWLER
THE REFRACTING TELESCOPE.—The function of a telescope is twofold.
First, to magnify the heavenly bodies, or, what comes to the same
thing, to make them look as if they were nearer to us, so that we can
see them better. Second, to collect a much greater number of rays
of light than the unassisted eye alone can grasp, so that objects
too dim to be otherwise perceptible are brought within our range of
vision.
There are two forms of telescope, distinguished as _Refractors_
and _Reflectors_. The simplest form of refracting telescope is
exemplified by the common opera-glass, and large refractors are not
essentially different. Such instruments depend for their action upon
the formation of an image by a lens. One can easily illustrate this
by producing upon the wall of a room an inverted image of a candle
or gas flame with a spectacle lens (one adapted for a long-sighted
person), or with one of the larger lenses from an opera-glass. Having
such an image, it may be magnified by means of another lens, just
as one may magnify a photograph with an ordinary reading glass.
Technically, the lens which forms the primary image is called the
_object-glass_ of the telescope, and that which is used to magnify
this image is called the _eye-piece_. The object-glass is usually a
large lens, which is placed at one end of a tube, while the eye-piece
is a much smaller lens, placed at the other end. Means are provided
for adjusting the distance between the two lenses so as to admit of
distinct vision.
Matters are, however, not quite so simple as has been stated. There
is a very great difficulty introduced by the fact that a lens made
out of a single piece of glass gives an image which is surrounded by
fringes of color, so that some device has to be adopted in order to
destroy, as far as possible, this enemy of good definition. In the
early history of the telescope, this so-called _chromatic aberration_
was considerably reduced by making small object-glasses of very great
focal length.[22]
Lenses of 100-foot focus, however, are not easy to employ as
object-glasses, and astronomy was, therefore, greatly benefited by
Dollond’s invention of the _achromatic lens_ in 1760. This is a
compound lens, usually consisting of a double convex crown-glass
lens and a concavo-convex, or double concave, lens of flint glass.
The curvatures of the lenses, and the optical properties of the two
kinds of glass composing them, are such that the color due to one
of them is practically neutralized by that due to the other acting
in opposition. A section of such an object-glass, with the “cell” in
which it rests, is shown in Fig. 20.
[Illustration: Fig. 20.—The Achromatic Object-Glass]
In this way the focal length of the lens, and, therefore, the length
of the telescope tube, can be kept within reasonable dimensions,
while the definition is improved. There is, however, usually a little
outstanding color, due to the imperfect matching of the two lenses,
and if one looks through a large refractor, even of a good quality,
a purple fringe will be noticed round all very bright objects. This
only affects a few of the brighter objects, while millions of others
which are dimmer may be seen free from spurious color.
It may be remarked that the curved surfaces of the lenses forming
telescopic object-glasses must not be parts of spheres. If they are,
the images will be rendered indistinct by _spherical aberration_, and
the optician has to design his curves to get rid of this defect at
the same time as chromatic aberration.
A new form of telescopic objective, consisting of three lenses,
which has many important advantages, has been invented by Mr. Dennis
Taylor, of the well-known firm of T. Cooke & Sons, York, England.
Such a lens as this illustrates the perfection which the optician’s
art has now attained. Six surfaces of glass have to be so accurately
figured that every ray of light falling upon the surface of the lens
shall pass through the finest pin-hole at a distance of eighteen
times the diameter of the lens.
THE REFLECTOR.—In a reflecting telescope, the object-glass of the
refractor is replaced by a concave mirror. In order that such a
mirror may reflect all the rays from a star to a single point, its
concave surface must be part of a paraboloid of revolution, that is,
a surface produced by the revolution of a parabola on its axis. If a
spherical surface be employed, all the rays will not be reflected to
a single point and the images which it gives will be ill-defined. Yet
it is astonishing to find that the difference between a parabolic and
spherical surface, even in the case of a large mirror, is exceedingly
small. Sir John Herschel states that in the case of a mirror four
feet in diameter, and forming an image at a distance of forty feet,
the parabolic only departs from the spherical form at the edges by
less than a twenty-one thousandth part of an inch.
[Illustration: Fig. 21.—The Newtonian Reflector]
An image being formed by a mirror, it is next to be viewed with
an eye-piece just as in the case of a refracting telescope. Here
there is a little difficulty, for if the eye-piece be applied in the
direct line of the mirror, the interposition of the observer’s head
will block out the light. Several ways of overcoming this have been
devised, but the plan most generally followed is that which Newton
adopted in the first reflecting telescope which was ever constructed.
With his own hands Newton made a small reflector, 6¼ inches long and
having an aperture of 1⅓ inches, with which he was able to study
the phases of Venus and the phenomena of Jupiter’s satellites. This
precious little instrument is now one of the greatest treasures in
the collection of the Royal Society of London. The general design
of this telescope is shown in Fig. 21. The concave mirror is at the
bottom of the telescope tube, and normally it would form an image of
a star near the end of the tube. A plane mirror, however, of small
size intercepts the rays and reflects them to the side, where they
converge to a focus. This image is observed and magnified by an
eye-piece, as in the refractor. It is true that in this arrangement
the plane mirror, or _flat_, renders the central part of the
principal mirror ineffective, but the loss of light is very much less
than would be the case if the eye-piece were placed in position to
view the image centrally.
In the hands of Sir William Herschel the reflecting telescope was
greatly developed. The great telescope with which he enriched
astronomical science had a mirror four feet in diameter, and its
tube was forty feet in length. With the view of utilizing the whole
surface of the mirror and dispensing with a second reflecting
surface, the 4-foot mirror was placed at a small angle to the bottom
of the tube, so that its principal focal point was no longer at the
centre, but at the side of the tube.
In practice, however, it is found that the Herschelian form of
reflector does not give the best definition, and it is now very
seldom seen.
Among other forms, the “Cassegrain” is perhaps the most important.
During the last years this form has received a great deal of
attention, more especially in regard to its special adaptability for
photographic purposes.
In the Cassegrain telescope, the plane mirror of the Newtonian form
is replaced by a small convex mirror which is part of a hyperboloid
of revolution, its axis and focal point being coincident with those
of the primary mirror. The rays are in this way reflected back to the
mirror at the bottom of the tube, and in order that the image may be
seen, it is necessary to cut out the middle part of the mirror to
admit the eye-piece.
Although the small mirror must theoretically be hyperbolic, tolerable
definition is obtained even if it be spherical or ellipsoidal, and
its actual departure from these forms is so slight as to be beyond
detection by measurement, so that the figuring of such mirrors can
only be tested in the telescope. For photographic purposes this
telescope has the very important advantage that a short telescope is
equivalent to a very long one of the Newtonian form, or refracting
telescope, so that the image of sun, moon, or planets formed at the
focus is very large in comparison with the size of the telescope. A
modification of this form of telescope, in which the small mirror
is out of the path of the rays falling upon the larger one, and no
longer obstructing the central part, has been revived by Dr. Common,
and has become generally known as the “Skew Cassegrain.”
In reflecting telescopes the mirrors were formerly made of speculum
metal (an alloy of copper and tin), and the word _speculum_ is even
now commonly employed to signify a telescopic mirror, although it is
usual to make the mirror of glass, with the concave surface silvered
and highly polished.
One is frequently asked for an opinion as to which is the better form
of telescope, the reflector or refractor, and it is a question that
one finds some little difficulty in answering. On one point, however,
all are agreed, namely, that the reflector has the advantage in
regard to its achromatism; it is indeed perfectly achromatic, while
the so-called “achromatic” refractor is at best only a compromise.
For the rest, one can not do better than quote the evidence of Dr.
Isaac Roberts before the International Astrophotographic Congress:
“The reflector requires the exercise of great care and patience, and
a thorough personal interest on the part of the observer using it. In
the hands of such a person it yields excellent results, but in other
hands it might be a bad instrument. The reflector gives results at
least equal, if not superior, to those obtained with the refractor,
if the observer be careful of the centring, and of the polish of the
mirror, and keeps the instrument in the highest state of efficiency;
but when intrusted to an ordinary assistant the conditions necessary
for its best performance can not be so well fulfilled as the same
could be in the case of the refractor.” One great practical advantage
of the reflector is that there are fewer optical surfaces, so that
a large reflector may be obtained for the price of a much smaller
refractor.
[Illustration: Fig. 22.—The Cassegrain Reflector]
EYE-PIECES.—So far we have regarded the eye-piece of a telescope as
a simple lens, but it is evident that the spherical and chromatic
aberration of such a lens will interfere with its performance. For
occasional use, however, even a simple lens is very serviceable if
the object observed is brought to the centre of the field of view.
Compound eye-pieces are of various forms, each having certain
advantages, the desiderata being freedom from color and “flatness
of field”—that is, stars in different parts of the field are to be
equally well in focus. Those most commonly employed are the Ramsden
and Huyghenian eye-pieces. The former consists of two plano-convex
lenses of equal focal lengths, having their curved faces toward each
other, and being placed at a distance apart equal to two-thirds of
the focal length of either lens. Such an eye-piece can be used as a
magnifying-glass, and it is therefore placed outside the focal image
formed by the telescope with which it is used; on this account it is
called a _positive eye-piece_. This kind of eye-piece is not quite
achromatic, but its flat field of view gives it a special value for
many purposes.
In the Huyghenian eye-piece there are again two lenses, made of
the same kind of glass. That which comes nearest to the eye has a
focal length of only one-third that of the _field_ lens, and the
distance between the two lenses is half the sum of the focal lengths.
This form of eye-piece can not be used as a magnifying-glass in
the ordinary sense, and as the field lens must be placed on the
object-glass or mirror side of the focus, it is called a _negative
eye-piece_. The Huyghenian eye-piece is more achromatic than the
Ramsden, and is more widely used when it is only required to view the
heavenly bodies. In instruments employed for purposes of measurement,
a positive eye-piece is essential in order that the spider threads
may be placed at the focus of the telescope. The images formed by an
astronomical telescope are upside down, and neither of the eye-pieces
described reinverts them.
A special form of eye-piece is therefore used when a telescope
is employed for terrestrial sight-seeing. The desired result is
obtained by the introduction of additional lenses, but there is a
corresponding reduction of brightness.
For viewing the sun some device is necessary to reduce the quantity
of light entering the eye. To look at the sun directly, even with a
small instrument, is very dangerous. The arrangement usually adopted
is a _solar diagonal_, in which the light is reflected from a piece
of plane glass before entering the eye-piece; the piece of glass
is wedge-shaped, so that the reflection from one surface only is
effective; if the glass had parallel sides, the solar image would be
double.
MAGNIFYING POWER.—The magnifying power of a telescope depends upon
the focal length of the object-glass, or speculum, and that of the
eye-piece. Optically, it is equal to the former divided by the
latter, so that the greater the focal length of an object-glass, or
the smaller the focal length of the eye-piece, the greater will be
the magnifying power. In a given telescope, the object-glass, or
speculum, is a constant factor and the magnifying power can only
be varied by changing the eye-piece. The focal length of the Lick
telescope, for example, is about 600 inches; with an eye-piece which
is equivalent to a lens of one-inch focus, the magnifying power
would be 600; with a lens of half an inch focus, it would be 1,200,
and so on.
The magnifying power which can be effectively employed, however,
depends upon a great variety of circumstances. First, the clearness
and steadiness of the air; then there is the quality of the
object-glass, or speculum, to be considered; and also the brightness
of the object to be observed, for when the object is very dim, its
light will be spread out into invisibility if too high a power be
used.
In practice, good refractors perform well with powers ranging up to
80 or 100 for each inch in the diameter of the object-glass. Thus, on
sufficiently bright objects, a six-inch telescope will work well with
a power of about 500, while a 30-inch may be effectively employed
with powers between 2,000 and 3,000.
ILLUMINATING POWER.—It has already been pointed out that
magnification is not the only function of a telescope. As a matter
of fact, the most powerful telescopes in the world fail to produce
the slightest increase in the apparent size of a star, for even if
these objects be brought to apparently a 3,000th part of their real
distances, they are still too far away to have any visible size. But
although a star can not be magnified, it can be rendered more visible
by the telescope, for the reason that the object-glass collects a
greater number of rays than the naked eye. The pupil of the eye may
be taken to have a diameter of one-fifth of an inch; a lens one-inch
in diameter will have twenty-five times the _area_ of the pupil, and
will therefore collect twenty-five times the amount of light from
a star; a two-inch lens will grasp one hundred times, and a 36-inch
32,400 times as much light as the pupil alone. Practically all these
rays collected by the object-glass, or speculum, of a telescope can
not be brought into the eye; some are lost through the imperfect
transparency of the glass, or the imperfect reflecting power of the
speculum. Still, allowing a considerable percentage for loss, there
is an enormous concentration of light when a large telescope is
employed.
THE ALTAZIMUTH MOUNTING.—Having got a telescope, we have next to
see how it can be best supported, for unless it be a very small
instrument indeed, it will be impossible to hold it in the hand like
a spy-glass. However a telescope be mounted, provision must be made
for turning it to any part of the sky whatsoever. Very frequently
one of the axes on which the instrument turns is vertical, while
the other is horizontal. Such a stand for a telescope is called an
_altazimuth mounting_, for the reason that it permits the instrument
to be moved in altitude and in azimuth.
As a rule, one finds only small telescopes mounted in this manner.
The objection to it is that, as one continues to observe a heavenly
body, two independent movements must be given to the telescope in
order to follow the body in its diurnal movement across the heavens.
If we commence observing a star newly risen, for example, the
telescope must trace a star-like path in order to follow it as it
ascends into the heavens.
THE EQUATORIAL TELESCOPE.—A much more convenient method of setting
up a telescope is to mount it as an _equatorial_. The essential
feature of this instrument is that one of the axes of movement,
instead of being vertical, is placed parallel to the axis of the
earth. This is called the _polar axis_, and, when the telescope is
turned around such an axis, it traces out curves in the sky which are
identical with those described by the stars in their diurnal motions.
If, then, the telescope be directed to a star or other heavenly body,
it can be made to follow the object and keep it in view by a single
movement. The axis at right angles to the polar axis is called the
declination axis, and is necessary in order that the telescope may be
moved toward and from the poles so that all the heavenly bodies above
the horizon may be included in its sweep.
One very important advantage of the equatorial is that, as only one
motion is required to keep a star in view, so long as it is above
the horizon, the necessary movement may be furnished by clockwork. A
good equatorial is accordingly provided with a driving-clock, which
is regulated so that it would drive the telescope through a whole
revolution once a day. Unlike an ordinary clock, the driving-clock of
a telescope is regulated by a governor, in order that the instrument
may have a continuous and not a jerky movement.
The telescope is also provided with clamps and fine adjustments, one
each in R. A. and declination, in order that it may be under the
control of the observer. It is evident that the telescope must be
capable of moving independently of the driving-gear, so that it may
first be placed in the desired direction; when this is accomplished,
the R. A. clamp is used to put the telescope in gear with the clock.
The declination clamp is then made to fix the telescope firmly to the
declination axis. Fine adjustments in both directions are necessary,
because it is impossible to sight a large instrument with such
precision as to bring an object exactly to the centre of the field of
view.
Some of the driving-clocks fitted to equatorials are very elaborate.
As clocks regulated by governors are not such reliable timekeepers
as those regulated by pendulums, arrangements are made by which the
accuracy of a pendulum can be electrically communicated to a governor
clock. One of the best forms of electrically controlled clocks is
that devised by Sir Howard Grubb.