MISCELLANEOUS REMARKS IN RELATION TO TELESCOPES.
The following remarks, chiefly in regard to the manner of using
telescopes, may perhaps be useful to young observers, who are not much
accustomed to the mode of managing these instruments.
1. _Adjustments requisite to be attended to in the use of telescopes._
When near objects are viewed with a considerable magnifying power, the
eye-tube requires to be removed farther from the object-glass than
when very distant objects are contemplated. When the telescope is
adjusted for an object, 6, 8, or 10 miles distant, a very considerable
alteration in the adjustment is requisite in order to see distinctly
an object at the distance of two or three hundred yards, especially if
the instrument is furnished with a high magnifying power. In this last
case, the eye-tube requires to be drawn out to a considerable distance
beyond the focus for parallel rays. I have found that, in a telescope
which magnifies 70 times, when adjusted for an object at the distance
of two miles, the adjustment requires to be altered fully one inch in
order to perceive distinctly an object at the distance of two or three
hundred yards; that is, the tube must be drawn, in this case, an inch
farther from the object-glass, and pushed in the same extent, when we
wish to view an object at the distance of two or three miles. These
adjustments are made, in pocket perspectives, by gently sliding the
eye-tube in or out, by giving it a gentle circular or spiral motion
till the object appear distinct. In using telescopes which are held
in the hand, the best plan is to draw all the tubes out to their full
length, and then, looking at the object, with the left hand supporting
the main tube near the object-glass, and the right supporting the
eye-tube--gently and gradually push in the eye-piece till distinct
vision be obtained. In Gregorian reflecting telescopes this adjustment
is made by means of a screw connected with the small speculum; and in
large achromatics, by means of a rack and pinion connected with the
eye-tube. When the magnifying power of a telescope is comparatively
small, the eye-tube requires to be altered only a very little.
There is another adjustment requisite to be attended to, in order to
adapt the telescope to the eyes of different persons. Those whose eyes
are too convex, or who are short-sighted, require the eye-tube to be
pushed in, and those whose eyes are somewhat flattened, as old people,
require the tube to be drawn out. Indeed there are scarcely two persons
whose eyes do not require different adjustments in a slight degree.
In some cases I have found that the difference of adjustment for two
individuals, in order to produce distinct vision in each, amounted
to nearly half an inch. Hence the difficulty of exhibiting the sun,
moon, and planets through telescopes, and even terrestrial objects,
to a company of persons who are unacquainted with the mode of using
or adjusting such instruments--not one half of whom generally see
the object distinctly--for, upon the proper adjustment of a telescope
to the eye, the accuracy of vision, in all cases, depends; and no one
except the individual actually looking through the instrument, can
be certain that it is accurately adjusted to his eye, and even the
individual himself, from not being accustomed to the view of certain
objects, may be uncertain whether or not the adjustment be correct. I
have found by experience that when the magnifying powers are high, as
150 or 200, the difference of adjustment required for different eyes
is very slight; but when low powers are used, as 20, 30, or 40, the
difference of the requisite adjustments is sometimes very considerable,
amounting to 1/4 or 1/2 of an inch.
2. _State of the Atmosphere most proper for observing terrestrial
and celestial objects._ The atmosphere which is thrown around the
globe--while it is essentially requisite to the physical constitution
of our world, and the comfort of its inhabitants--is found in
many instances a serious obstruction to the accurate performance
of telescopes. Sometimes it is obscured by mists and exhalations,
sometimes it is thrown into violent undulations by the heat of the
sun and the process of evaporation, and even, in certain cases, where
there appears a pure unclouded azure, there is an agitation among its
particles and the substances incorporated with them, which prevents
the telescope from producing distinct vision either of terrestrial or
celestial objects. For viewing distant terrestrial objects, especially
with high powers, the best time is early in the morning, a little
after sun-rise, and, from that period till about 9 o’clock A.M., in
summer; and, in the evening about two or three hours before sun-set.
From about 10 o’clock A.M. till 4 or 5 in the afternoon, in
summer, if the sky be clear and the sun shining, there is generally
a considerable undulation in the atmosphere, occasioned by the solar
rays and the rapid evaporation, which prevents high powers from being
used with distinctness on any telescope, however excellent. The objects
at such times, when powers of 50, 70, or 100 are applied, appear to
undulate like the waves of the sea, and, notwithstanding every effort
to adjust the telescope, they appear confused and indistinct. Even
with very moderate magnifying powers this imperfection is perceptible.
In such circumstances, I have sometimes used a power of 200 times
on distant land objects, with good effect, a little before sun-set,
when, in the forenoon of the same day, I could not have applied a
power of 50 with any degree of distinctness. On days when the air is
clear, and the atmosphere covered with clouds, terrestrial objects
may be viewed with considerably high powers. When there has been a
long-continued drought, the atmosphere is then in a very unfit state
for enjoying distinct vision with high magnifying powers, on account of
the quantity of vapours with which the atmosphere is then surcharged,
and the undulations they produce. But, after copious showers of rain,
especially if accompanied with high winds, the air is purified, and
distant objects appear with greater brilliancy and distinctness than at
any other seasons. In using telescopes, the objects at which we look
should, if possible, be nearly in a direction opposite to that of the
sun. When they are viewed nearly in the direction of the sun, their
shadows are turned towards us, and they consequently appear dim and
obscure. By not attending to this circumstance, some persons, in trying
telescopes, have pronounced a good instrument to be imperfect, which,
had it been tried on objects properly illuminated, would have been
found to be excellent. In our variable northerly climate the atmosphere
is not so clear and serene for telescopic observation as in Italy,
the South of France, and in many of the countries which lie within
the tropics. The undulations of the air, owing to the causes alluded
to above, constitute one of the principal reasons why a telescope
magnifying above a hundred times can seldom be used with any good
effect in viewing terrestrial objects--though I have sometimes used a
power of nearly 200 with considerable distinctness, in the stillness
of a summer or autumnal evening, when the rays of the declining sun
strongly illuminated distant objects.
The atmosphere is likewise frequently a great obstruction to the
distinct perception of _celestial_ objects. It is scarcely possible for
one who has not been accustomed to astronomical observations, to form
a conception of the very great difference there is in the appearance
of some of the heavenly bodies in different states of the atmosphere.
There are certain conditions of the atmosphere essentially requisite
for making accurate observations with powerful telescopes, and it
is but seldom, especially in our climate, that all the favourable
circumstances concur. The nights must be very clear and serene--the
moon absent--no twilight--no haziness--no violent wind--no sudden
change of temperature, as from thaw to frost--and no surcharge of the
atmosphere with aqueous vapour. I have frequently found that, on the
first and second nights after a thaw, when a strong frost had set in,
and when the heavens appeared very brilliant, and the stars vivid
and sparkling--the planets, when viewed with high powers, appeared
remarkably undefined and indistinct; their margins appeared waving and
jagged, and the belts of Jupiter, which at other times were remarkably
distinct, were so obscured and ill-defined, that they could with
difficulty be traced. This is probably owing to the quantity of aqueous
vapour, and perhaps icy particles, then floating in the air, and to
the undulations thereby produced. When a hard frost has continued a
considerable time, this impediment to distinct observation is in a
great measure removed. But I have never enjoyed more accurate and
distinct views of the heavenly bodies than in fresh serene evenings,
when there was no frost and no wind, and only a few fleecy clouds
occasionally hovering around. On such evenings, and on such alone, the
highest powers may be applied. I have used magnifying powers on such
occasions with good effect, which could not have been applied, so as to
ensure distinct vision, more frequently than two or three days in the
course of a year.
Sir William Herschel has observed, in reference to this point, ‘In
beautiful nights, when the outside of our telescopes is dropping with
moisture, discharged from the atmosphere, there are now and then
favourable _hours_ in which it is hardly possible to put a limit to
the magnifying powers. But such valuable opportunities are extremely
scarce, and with large instruments it will always be lost labour to
observe at other times. In order therefore, to calculate how long a
time it must take to sweep the heavens, as far as they are within
the reach of my forty-feet telescope, charged with a magnifying
power of 1000, I have had recourse to my journals to find how many
favourable hours we may annually hope for in this climate. And, under
all favourable circumstances, it appears, that a year which will
afford ninety, or at most, one hundred _hours_ is to be called very
productive.’ ‘In the equator, with my twenty feet telescope, I have
swept over zones of two degrees with a power of 157, but an allowance
of ten minutes in Polar distance must be made for lapping the sweeps
over one another where they join. As the breadth of the zones may be
increased towards the poles, the northern hemisphere may be swept in
about 40 zones; to these we must add 19 southern zones; then 59 zones
which, on account of the sweeps lapping over one another, about 5
minutes of time in right ascension, we must reckon of 25 hours each,
will give 1475 hours. And allowing 100 hours per year, we find that
with the 20 feet telescope, the heavens may be swept in about 14 years
and three quarters. Now the time of sweeping with different magnifying
powers will be as the squares of the powers; and putting _p_ and _t_
for the power and time in the 20 feet telescope, and P = 1000 for
the power in the 40 feet instrument, we shall have p^2 : t :: P^2 :
^{tP2}/_{p^2} = 59840. Then making the same allowance for 100 hours per
year, it appears that it will require not less than 598 years, to look
with the 40 feet reflector, charged with the above-mentioned power,
only one single moment into each point of space; and even then, so much
of the southern hemisphere will remain unexplored, as will take up 213
years more to examine.’[28]
From the above remarks of so eminent an observer, the reader will
perceive how difficult it is to explore the heavens with minuteness and
accuracy, and with how many disappointments, arising from the state
of the atmosphere, the astronomer must lay his account, when employed
in planetary or sidereal investigation. Besides the circumstances now
stated, it ought to be noticed that a star or a planet is only in
a situation for a high magnifying power, about half the time it is
above the horizon. The density of the atmosphere, and the quantity of
vapours with which it is charged near the horizon, prevent distinct
vision of celestial objects with high powers, till they have risen to
at least 15 or 20 degrees in altitude, and the highest magnifiers can
scarcely be applied with good effect, unless the object is near the
meridian, and at a considerable elevation above the horizon. If the
moon be viewed a little after her rising, and afterwards when she comes
to her highest elevation in autumn, the difference in her appearance
and distinctness will be strikingly perceptible. It is impossible to
guess whether a night be well adapted for celestial observations,
till we actually make the experiment, and instruments are frequently
condemned, when tried at improper seasons, when the atmosphere only is
in fault. A certain observer remarks,--‘I have never seen the face of
Saturn more distinctly than in a night when the air has been so hazy,
that with my naked eye, I could hardly discern a star of less than the
third magnitude.’ The degree of the transparency of the air is likewise
varying almost in the course of every minute, so that even in the
course of the same half hour, planets and stars will appear perfectly
defined, and the reverse. The vapours moving and undulating the
atmosphere, even when the sky appears clear to the naked eye, will in a
few instants destroy the distinctness of vision, and in a few seconds
more, the object will resume its clear and well-defined aspect.[29]
3. _On the magnifying powers requisite for observing the phenomena of
the different planets--comets--double stars, &c._
There are some objects connected with astronomy which cannot be
perceived without having recourse to instruments and to powers of
great magnitude. But it is a vulgar error to imagine that very large
and very expensive telescopes are absolutely necessary for viewing
the greater part of the more interesting scenery of the heavens. Most
of the phenomena of the planets, comets and double stars and other
objects, are visible with instruments of moderate dimensions, so that
every one who has a relish for celestial investigations, may, at a
comparatively small expense, procure a telescope, for occasional
observations, which will show the principal objects and phenomena
described in books on astronomy. Many persons have been misled by some
occasional remarks which Sir W. Herschel made, in reference to certain
very high powers which he sometimes put, by way of experiment, on some
of his telescopes, as if these were the powers requisite for viewing
the objects to which he refers. For example, it is stated that he once
put a power of 6450 times on his 7 feet Newtonian telescope of 6-3/10
inches aperture; but this was only for the purpose of an experiment,
and could be of no use whatever when applied to the moon, the planets
and most objects in the heavens. Herschel, through the whole course of
his writings, mentions his only having used it _twice_, namely on the
stars α Lyræ, and γ Leonis, which stars can be seen more distinctly and
sharply defined with a power of 420. To produce a power of 6450 on such
a telescope, would require a lens of only 1/77th of an inch in focal
distance, and it is questioned by some whether Herschel had lenses of
so small a size in his possession, or whether it is possible to form
them with accuracy.
_Powers requisite for observing the phenomena of the planets._--The
planet Mercury requires a considerable magnifying power, in order to
perceive its phases with distinctness. I have seldom viewed this planet
with a less power than 100 and 150, with which powers its half moon,
its gibbous, and its crescent phase, may be distinctly perceived. With
a power of 40, 50, or even 60 times, these phases can with difficulty
be seen, especially as it is generally at a low altitude, when such
observations are made. The phases of _Venus_ are much more easily
distinguished, especially the _crescent_ phase, which is seen to
the greatest advantage about a month before and after the inferior
conjunction. With a power not exceeding 25 or 30 times, this phase,
at such periods, may be easily perceived. It requires, however, much
higher powers to perceive distinctly the variations of the gibbous
phase; and if this planet be not viewed at a considerably high altitude
when in a half-moon or gibbous phase, the obscurity and undulations
of the atmosphere near the horizon, prevent such phases from being
accurately distinguished, even when high powers are applied. Although
certain phenomena of the planets may be seen with such low powers as
I have now stated, yet, in every instance, the highest magnifying
powers, consistent with distinctness, should be preferred, as the eye
is not then strained, and the object appears with a greater degree of
magnitude and splendour. The planet _Mars_ requires a considerable
degree of magnifying power, even when at its nearest distance from the
earth, in order to discern its spots and its gibbous phase. I have
never obtained a satisfactory view of the spots which mark the surface,
and their relative position, with a less power than 130, 160, or 200
times; and even with such powers, persons not much accustomed to look
through telescopes, find a difficulty in distinguishing them.
The strongest and most prominent _belts of Jupiter_, may be seen with
a power of about 45; which power may be put upon a 20-inch achromatic,
or a 1 foot reflector. But a satisfactory view of all the belts, and
the relative positions they occupy, cannot be obtained with much
lower powers than 80, 100, or 140. The most common positions of these
belts are--one dark and well-defined belt to the south of Jupiter’s
equator; another of nearly the same description to the north of it,
and one about his north and his south polar circles. These polar belts
are much more faint, and consequently not so easily distinguished as
the equatorial belts. The _moons_ of this planet, in a very clear
night, may sometimes be seen with a pocket 1 foot achromatic glass,
magnifying about 15 or 16 times. Some people have pretended that they
could see some of these satellites with their naked eye; but this is
very doubtful, and it is probable that such persons mistook certain
fixed stars which happened to be near Jupiter for his satellites. But,
in order to have a clear and interesting view of these, powers of at
least 80 or 100 times should be used. In order to perceive their
immersions into the shadow of Jupiter, and the exact moment of their
emersions from it, a telescope not less than a 44 inch achromatic, with
a power of 150 should be employed. When these satellites are viewed
through large telescopes with high magnifying powers, they appear
with well defined disks, like small planets. The planet Jupiter has
generally been considered as a good test by which to try telescopes
for celestial purposes. When it is near the meridian and at a high
altitude, if its general surface, its belts, and its margin appear
distinct and well-defined, it forms a strong presumptive evidence that
the instrument is a good one.
The planet _Saturn_ forms one of the most interesting objects for
telescopic observation. The _ring_ of Saturn may be _seen_ with a power
of 45; but it can only be contemplated with advantage when powers
of 100, 150, and 200 are applied to a 3 or a 5 feet achromatic. The
_belts of Saturn_ are not to be seen distinctly with an achromatic
of less than 2-3/4 inches aperture, or a Gregorian reflector of less
than 4 inches aperture, nor with a less magnifying power than 100
times. Sir W. Herschel has drawn this planet with five belts across
its disk; but it is seldom that above one or two of them can be seen
by moderate-sized telescopes and common observers. The _division_
of the double ring, when the planet is in a favorable position for
observation, and in a high altitude, may sometimes be perceived with
a 44-inch achromatic, with an aperture of 2-3/4 inches, and with
powers of 150 or 180, but higher powers and larger instruments are
generally requisite to perceive this phenomenon distinctly; and even
when a portion of it is seen at the extremities of the _ansæ_, the
division cannot, in every case, be traced along the whole of the
half-circumference of the ring which is presented to our eye. Mr.
Hadley’s engraving of Saturn, in the ‘Philosophical Transactions’
for 1723, though taken with a Newtonian reflector with a power of
228, represents the division of the ring as seen only on the ansæ or
extremities of the elliptic figure in which the ring appears. The
best period for observing this division is when the ring appears at
its utmost width. In this position it was seen in 1840, and it will
appear nearly in the same position in 1855. When the ring appears like
a very narrow ellipse, a short time previous to its disappearance, the
division, or dark space between the rings, cannot be seen by ordinary
instruments.
Sir W. Herschel very properly observes, ‘There is not perhaps another
object in the heavens that presents us with such a variety of
extraordinary phenomena as the planet Saturn; a magnificent globe,
encompassed by a stupendous double ring; attended by seven satellites;
ornamented with equatorial belts; compressed at the poles; turning upon
its axis; mutually eclipsing its ring and satellites, and eclipsed by
them; the most distant of the rings also turning upon its axis, and the
same taking place with the farthest of the satellites; all the parts
of the system of Saturn occasionally reflecting light on each other;
the rings and moons illuminating the nights of the Saturnian, the globe
and satellites enlightening the dark parts of the ring; and the planet
and rings throwing back the sun’s beams upon the moons, when they are
deprived of them at the time of their conjunctions.’ This illustrious
astronomer states, that with a new 7 feet mirror of extraordinary
distinctness he examined this planet, and found that the ring reflects
more light than the body, and with a power of 570 the colour of the
body becomes yellowish, while that of the ring remains more white. On
March 11, 1780, he tried the powers of 222, 332, and 440 successively,
and found the light of Saturn less intense than that of the ring; the
colour of the body turning, with the high powers, to a kind of yellow
white, while that of the ring still remained white.
Most of the _satellites_ of Saturn are difficult to be perceived
with ordinary telescopes, excepting the 4th, which may be seen with
powers of from 60 to 100 times. It was discovered by Huygens in 1655,
by means of a common refracting telescope 12 feet long, which might
magnify about 70 times. The next in brightness to this is the 5th
satellite, which Cassini discovered in 1671, by means of a 17 feet
refractor, which might carry a power of above 80 times. The 3rd was
discovered by the same astronomer in 1672, by a longer telescope; and
the 1st and 2nd, in 1684, by means of two excellent object-glasses of
100 and 136 feet, which might have magnified from 200 to 230 times.
They were afterwards seen by two other glasses of 70 and 90 feet, made
by Campani, and sent from Rome to the Royal Observatory at Paris, by
the King’s order, after the discovery of the 3rd and 5th satellites.
It is asserted, however, that all those 5 satellites were afterwards
seen with a telescope of 34 feet, with an aperture of 3-3/10 inches,
which would magnify about 120 times. These satellites, on the whole,
except the 4th and 5th, are not easily detected. Dr. Derham, who
frequently viewed Saturn through Huygens’ glass of 126 feet focal
length, declares, in the preface to his ‘Astro-Theology,’ that he could
never perceive above 3 of the satellites. Sir W. Herschel observes,
that the visibility of these minute and extremely faint objects,
depends more on the _penetrating_ than upon the _magnifying_ power
of our telescopes; and that with a 10 feet Newtonian, charged with
a magnifying power of only 60, he saw all the 5 old satellites; but
the 6th and 7th, which were discovered and were easily seen with his
40-feet telescope, and were also visible in his 20-feet instrument,
were not discernible in the 7 or the 10-feet telescopes, though all
that _magnifying power_ can do may be done as well with the 7-feet as
with any larger instrument. Speaking of the 7th satellite, he says,
‘Even in my 40-feet reflector it appears no bigger than a very small
lucid point. I see it, however, very well in the 20-feet reflector; to
which the exquisite figure of the speculum not a little contributes.’
A late observer asserts, that in 1825, with a 12-feet achromatic, of 7
inches aperture, made by Tulley, with a power of 150, the 7 satellites
were easily visible, but not so easily with a power of 200; and that
the planet appeared as bright as brilliantly burnished silver, and the
division in the ring and a belt were very plainly distinguished, with a
power of 200.
The planet _Uranus_, being generally invisible to the naked eye, is
seldom an object of attention to common observers. A considerable
magnifying power is requisite to make it appear in a planetary form
with a well-defined disk. The best periods for detecting it are, when
it is near its opposition to the sun, or when it happens to approximate
to any of the other planets, or to a well-known fixed star. When none
of these circumstances occur, its position requires to be pointed out
by an Equatorial Telescope. On the morning of the 25th January, 1841,
this planet happened to be in conjunction with Venus, at which time
it was only 4 minutes north of that planet. Several days before this
conjunction, I made observations on Uranus. On the evening of the
24th, about 8 hours before the conjunction, the two planets appeared
in the same field of the telescope, the one exceedingly splendid, and
the other more obscure, but distinct and well-defined. Uranus could
not be perceived, either with the naked eye, or with an opera glass;
but could be distinguished as a very small star by means of a pocket
achromatic telescope magnifying about 14 times. It is questionable
whether, under the most favourable circumstances, this planet can ever
be distinguished by the naked eye. With magnifying powers of 30 and 70,
it appeared as a moderately large star with a steady light, but without
any sensible disk. With powers of 120, 180, and 250, it presented a
round and pretty well-defined disk, but not so luminous and distinct as
it would have done in a higher altitude.
The _Double Stars_ require a great _variety_ of powers, in order to
distinguish the small stars that accompany the larger. Some of them
are distinguished with moderate powers, while others require pretty
large instruments, furnished with high magnifying eye-pieces. I shall
therefore select only a few as a specimen. The star _Castor_, or α
Geminorum, may be easily seen to be double with powers of from 70 to
100. I have sometimes seen these stars, which are nearly equal in size
and colour, with a terrestrial power of 44 on a 44-inch achromatic. The
appearance of this star with such powers is somewhat similar to that
of η Coronæ in a 7 feet achromatic, of 5 inches aperture, with a power
of 500. γ Andromedæ may be seen with a moderate power. In a 30-inch
achromatic of 2 inches aperture, and a power of 80, it appears like
ε Bootis, when seen in a 5-feet achromatic, with a power of 460. This
star is said to be visible even in a 1-foot achromatic with a power of
35. ε _Lyræ_, which is a quintuple star, but appears to the naked eye
as a single star,--may be seen to be double with a power of from 6 to
12 time. γ _Leonis_ is visible in a 44-inch achromatic, with a power of
180 or 200. _Rigel_ in a 3-1/2-feet achromatic, may be seen with powers
varying from 130 to 200. The small star, however, which accompanies
Rigel, is sometimes difficult to be perceived, even with such powers.
ε _Bootis_ is seldom distinctly defined with an achromatic of less
aperture than 3-1/4 inches, or a reflector of less than 5 inches, with
a power of at least 250.
These and similar stars are not to be expected to be seen equally well
at all times, even when the magnifying and illuminating powers are
properly proportioned; as much depends upon the state of the weather,
and the pureness of the atmosphere. In order to perceive the closest
of the double stars, Sir W. Herschel recommends, that the power of the
telescope should be adjusted upon a star known to be single, of nearly
the same altitude, magnitude, and colour with the double star which
is to be observed, or upon one star above and another below it. Thus,
the late Mr. Aubert, the astronomer, could not see the two stars of γ
Leonis, when the focus was adjusted upon that star itself; but he soon
observed the small star, after he had adjusted the focus upon Regulus.
An exact adjustment of the focus of the instrument is indispensably
requisite, in order to perceive such minute objects.
In viewing the _Nebulæ_, and the very small and immensely distant
fixed stars, which require much light to render them visible, a large
aperture of the object-glass or speculum, which admits of a great
quantity of light, is of more importance than high magnifying powers.
It is light chiefly, accompanied with a moderate magnifying power,
that enables us to _penetrate_ into the distant regions of space. Sir
W. Herschel, when sweeping the profundities of the Milky way, and the
Hand and Club of Orion, used a telescope of the Newtonian form, 20-feet
focal length, and 18-7/10 inches diameter, with a power of only 157.
On applying this telescope and power to a part of the _Via Lactea_, he
found that it completely resolved the whole whitish appearance into
stars, which his former telescopes had not light enough to effect; and
which smaller instruments with much higher magnifying powers would
not have effected. He tells us, that with this power, ‘the glorious
multitude of stars,’ in the vicinity of Orion, ‘of all possible sizes,
that presented themselves to view, was truly astonishing, and that
he had fields which contained 70, 90 and 110 stars, so that a belt
of 15 degrees long, and 2 degrees broad, which passed through the
field of the telescope in an hour, could not contain less than fifty
thousand stars that were large enough to be distinctly numbered.’ In
viewing the Milky way, the Nebulæ, and small clusters of stars, such
as _Præsepe_ in Cancer, I generally use a power of 55 times, on an
achromatic telescope 6 feet 6 inches in focal length, and 4 inches
diameter. The eye-piece, which produces this power--which I formed
for the purpose--consists of two convex lenses, the one next the eye
3 inches focal length, and 1-2/10 inch diameter, and that next the
object 3-1/2 inches focus, and 1-4/10 inch diameter, the deepest convex
surfaces being next each other, and their distance 1/4 inch. With this
eye-piece a very large and brilliant field of view is obtained; and I
find it preferable to any higher powers in viewing the nebulosities,
and clusters of stars. In certain spaces of the heavens, it sometimes
presents in one field, nearly a hundred stars. It likewise serves to
exhibit a very clear and interesting view of the full moon.
In observing _Comets_, a very small power should generally be used,
even on large instruments. These bodies possess so small a quantity
of light, and they are so frequently enveloped in a veil of dense
atmosphere, that _magnifying_ power sometimes renders them more
obscure; and therefore the _illuminating_ power of a large telescope,
with a small power, is in all cases to be preferred. A comet eye-piece
should be constructed with a very large and uniformly distinct field,
and should magnify only from 15 to 30 or 40 times, and the lenses of
such an eye-tube should be nearly two inches in diameter. The late
Rev. F. Wollaston recommended for observing comets, ‘a telescope with
an achromatic object-glass of 16 inches focal length, and 2 inches
aperture, with a Ramsden’s eye-glass magnifying about 25 times, mounted
on a very firm equatorial stand, the field of view taking in 2 degrees
of a great circle.’
In viewing the _moon_, various powers may be applied according to
circumstances. The best periods of the moon for inspecting the
inequalities on its surface, are either when it assumes a crescent or
a half-moon phase, or two or three days after the period of half-moon.
Several days after full-moon, and particularly about the third quarter,
when this orb is waning, and when the shadows of its mountains and
vales are thrown in a different direction from what they are when
on the increase,--the most prominent and interesting views may be
obtained. The most convenient season for obtaining such views is during
the autumnal months, when the moon, about the third quarter, sometimes
rises as early as 8 o’clock P.M., and may be viewed at a
considerably high altitude by ten or eleven. When in the positions now
alluded to, and at a high altitude, very high magnifying powers may
sometimes be applied with good effect, especially if the atmosphere be
clear and serene. I have sometimes applied a power, in such cases, of
350 times, on a 46-inch achromatic, with considerable distinctness;
but it is only two or three times in a year, and when the atmosphere
is remarkably favourable, that such a power can be used. The autumnal
evenings are generally best fitted for such observations. The _full
moon_ is an object which is never seen to advantage with high powers,
as no shadows or inequalities on its surface can then be perceived.
It forms, however, a very beautiful object, when magnifying powers
not higher than 40, 50, or 60 times are used. A power of 45 times, if
properly constructed, will show the _whole of the moon_ with a margin
around it, when the darker and brighter parts of its surface will
present a variegated aspect, and appear somewhat like a map to the eye
of the observer.
4. _Mode of exhibiting the Solar spots._
The solar spots may be contemplated with advantage by magnifying powers
varying from 60 to 180 times; about 90 times is a good medium power,
though they may sometimes be distinguished with very low powers, such
as those usually adapted to a one-foot telescope, or even by means of a
common opera-glass. The common astronomical eye-pieces given along with
achromatic telescopes, and the sun-glasses connected with them, are
generally ill-adapted for taking a pleasant and comprehensive view of
the solar spots. In the higher magnifying powers, the first eye-glass
is generally at too great a distance from the eye, and the sun-glass
which is screwed over it, removes it to a still greater distance from
the point to which the eye is applied, so that not above one third
of the field of view can be taken in. This circumstance renders it
difficult to point the instrument to any particular small spot on the
solar disk which we wish minutely to inspect; and besides, it prevents
us from taking a comprehensive view of the _relative positions_ of
all the spots that may at any time be traversing the disk. To obviate
this inconvenience, the sun-glass would require to be placed so near
to the glass next the eye as almost to touch it. But this is sometimes
difficult to be attained, and, in high powers, even the thickness of
the sun-glass itself is sufficient to prevent the eye from taking in
the whole field of view. For preventing the inconveniences to which
I now allude, I generally make use of a _terrestrial_ eye-piece of a
considerable power, with a large field, the sun-glass is fixed at the
end of a short tube which slides on the eye-piece, and permits the
coloured glass to approach within a line or two of the lens next the
eye, so that the whole field of the telescope is completely secured.
The eye-piece alluded to carries a magnifying power of 95 times for
a 46-inch telescope, and takes in about three fourths of the surface
of the sun, so that the relative positions of all the spots may
generally be perceived at one view. Such a power is, in most cases,
quite sufficient for ordinary observations; and I have seldom found any
good effect to arise from attempting very high powers, when minutely
examining the solar spots.
But, the most pleasant mode of viewing the solar spots--especially when
we wish to exhibit them to others--is to throw the image of the sun
upon a white screen, placed in a room which is considerably darkened.
It is difficult, however, when the sun is at a high altitude, to put
this method into practice, on account of the great obliquity with
which his rays then fall, which prevents a screen from being placed
at any considerable distance from the eye-end of the telescope. The
following plan, therefore, is that which I uniformly adopt as being
both the easiest and the most satisfactory. A telescope is placed in a
convenient position, so as to be directed to the sun. This telescope
is furnished with a _diagonal eye-piece_, such as that represented,
fig. 77, (p. 344.) The window-shutters of the apartment are all closed,
excepting a space sufficient to admit the solar rays; and, when the
telescope is properly adjusted, a beautiful image of the sun, with
all the spots which then happen to diversify his surface, is thrown
upon the _ceiling_ of the room. This image may be from 12 to 20, or 30
inches or more in diameter, according to the distance of the ceiling
from the diagonal eye-piece. The greater this distance is, the larger
the image. If the sun is at a very high altitude, the image will be
elliptical; if he be at no great distance from the horizon, the image
will appear circular or nearly so; but in either case the spots will be
distinctly depicted, provided the focus of the telescope be accurately
adjusted. In this exhibition, the apparent motion of the sun, produced
by the rotation of the earth, and the passage of thin fleeces of clouds
across the solar disk, exhibit a very pleasing appearance.
By this mode of viewing the solar spots we may easily ascertain their
diameter and magnitude, at least to a near approximation. We have only
to take a scale of inches, and measure the diameter of any well-defined
and remarkable spot, and then the diameter of the solar image; and,
comparing the one with the other, we can ascertain the number of miles
either lineal or square, comprehended in the dimensions of the spot.
For example, suppose a spot to measure one half-inch in diameter,
and the whole image of the sun 25 inches, the proportion between
the diameter of the spot and that of the sun will be as 1 to 50, in
other words, the _one fiftieth_ part of the sun’s diameter. Now, this
diameter being 880,000 miles, this number, divided by 50, produces a
quotient of 17,600 = the number of miles which its diameter measures.
Such a spot will therefore contain an area of 243,285,504, or more
than two hundred and forty-three millions of square miles, which is 46
millions of miles more than the whole superficies of the terraquous
globe. Again, suppose the diameter of a spot measures 3/10 inch, and
the solar image 23 inches, the proportion of the diameter of the spot
to that of the sun is as 3 to 230 = the number of tenths in 23 inches.
The number of miles in the spot’s diameter will therefore be found
by the following proportion: 230 : 880,000 :: 3 : 11,478; that is,
the diameter of such a spot measures eleven thousand four hundred and
seventy-eight miles. Spots of such sizes are not unfrequently seen to
transit the solar disk.
By this mode of viewing the image of the sun, his spots may be
exhibited to twenty or thirty individuals at once without the least
straining or injury to the eyes; and as no separate screen is
requisite, and as the ceilings of rooms are generally white, the
experiment may be performed in half a minute without any previous
preparation, except screwing on and adjusting the eye-piece. The manner
of exhibiting the solar spots, in this way, is represented in fig. 82.
[Illustration: _figure 82._]
5. _On the space-penetrating power of telescopes._--The power of
telescopes to penetrate into the profundity of space is the result of
the quantity of light they collect and send to the eye in a state fit
for vision. This property of telescopes is sometimes designated by the
expression _Illuminating Power_.
Sir W. Herschel appears to have been the first who made a distinction
between the _magnifying_ power, and the _space-penetrating_ power
of a telescope; and there are many examples which prove that such
a distinction ought to be made, especially in the case of large
instruments. For example, the small star, or speck of light, which
accompanies the pole-star, may be seen through a telescope of large
aperture, with a smaller magnifying power than with a telescope of a
small aperture furnished with a much higher power. If the magnifying
power is sufficient to show the small star completely separated from
the rays which surround the large one, this is sufficient in one point
of view; but in order that this effect may be produced, so as to render
the small star perfectly distinguishable, a certain quantity of light
must be admitted into the pupil of the eye--which quantity depends
upon the area of the object-glass or speculum of the instrument, or,
in other words, on the illuminating power. If we compare a telescope
of 2-3/4 inches aperture with one of 5 inches aperture, when the
magnifying power of each does not exceed 50 times for terrestrial
objects, the effect of illuminating power is not so evident; but
if we use a power of 100 for day objects, and 180 for the heavenly
bodies, the effects of illuminating power is so clearly perceptible,
that objects not only appear brighter, and more clearly visible, in
the larger telescope, but with the same magnifying power, they also
_appear_ larger, particularly when the satellites of Jupiter and small
stars are the objects we are viewing.
Sir W. Herschel remarks, that ‘objects are viewed in their greatest
perfection, when, in penetrating space, the magnifying power is so
low as only to be sufficient to show the object well--and when,
in magnifying objects, by way of examining them minutely, the
space-penetrating power is no higher than what will suffice for the
purpose; for in the use of either power, the injudicious overcharge
of the other will prove hurtful to vision.’ When illuminating power is
in too high a degree, the eye is offended by the extreme brightness of
the object. When it is in too low a degree, the eye is distressed by
its endeavours to see what is beyond its reach; and therefore it is
desirable, when we wish to give the eye all the assistance possible, to
have the illuminating and the magnifying powers in due proportion. What
this proportion is, depends, in a certain degree, upon the brightness
of the object. In proportion to its brightness or luminosity, the
magnifying power may, to a certain extent, be increased. Sir W.
Herschel remarks, in reference to α Lyræ, ‘This star, I surmise, has
light enough to bear being magnified, at least a hundred thousand
times, with no more than six inches of aperture.’ However beautifully
perfect any telescopes may appear, and however sharp their _defining_
power, their performance is limited by their illuminating powers--which
are as the squares of the diameters of the apertures of the respective
instruments. Thus, a telescope whose object-glass is 4 inches diameter
will have four times the quantity of light, or illuminating power,
possessed by a telescope whose aperture is only 2 inches, or in the
proportion of 16 to 4,--the square of 4 being 16, and the square of 2
being 4.
The nature of the _space-penetrating power_, to which we are
adverting, and the distinction between it, and magnifying power,
may be illustrated from a few examples taken from Sir W. Herschel’s
observations.
The first observation which I shall notice refers to the _nebula_
between η and ζ Ophiuchi, discovered by Messier in 1764. The
observation was made with a 10 feet reflector, having a magnifying
power of 250, and a space-penetrating power of 28.67. His note is dated
May 3, 1783. ‘I see several stars in it, and make no doubt a higher
power and more light will resolve it all into stars. This seems to me
a good nebula for the purpose of establishing the connection between
nebulæ and clusters of stars in general.’--‘June 18, 1784. The same
nebula viewed with a Newtonian 20 feet reflector; penetrating power 61,
and a magnifying power of 157; a very large and a very bright cluster
of excessively compressed stars. The stars are but just visible, and
are of unequal magnitudes. The large stars are red, the cluster is a
miniature of that near Flamstead’s forty-second Comæ Berenices; Right
ascension 17^h 6^m 32^s Polar distance 108° 18´´’ In this case, a
penetrating power of about 28, with a magnifying power of 250, barely
shewed a few stars; when in the second instrument the illuminating
power of 60 with the magnifying power of only 157 showed them
completely.
Subsequently to the date of the latter observation, the 20 feet
Newtonian telescope was converted into an Herschelian instrument, by
taking away the small speculum, and giving the large one the proper
inclination for obtaining the front view; by which alteration the
illuminating power was increased from 61 to 75, and the advantage
derived from the alteration was evident in the discovery of the
satellites of Uranus by the altered telescope, which before was
incompetent in the point of penetration, or illuminating power. ‘March
14, 1798, I viewed the Georgian planet (or Uranus) with a new 25 feet
reflector. Its penetrating power is 95.85, and having just before
also viewed it with my 20 feet instrument, I found that with an equal
magnifying power of 300, the 25 feet telescope had considerably the
advantage of the former.’ The aperture of the 20 feet instrument was
18.8 inches, and that of the 25 feet telescope, 24 inches, so that
the superior effect of the latter instrument must have been owing
to its greater illuminating power. The following observations show
the superior power of the 40 feet telescope as compared with the 20
feet.--‘Feb. 24, 1786, I viewed the nebula near Flamstead’s fifth
Serpentis, with my 20 feet reflector, magnifying power 157. The most
beautiful extremely compressed cluster of small stars; the greatest
part of them gathered together into one brilliant nucleus, evidently
consisting of stars, surrounded with many detached gathering stars of
the same size and colour. R.A. 15^h 7^m 12^s. P.D. 87° 8´´’--‘May 27,
1791, I viewed the same object with my 40 feet telescope, penetrating
power 191.69, magnifying power 370. A beautiful cluster of stars. I
counted about 200 of them. The middle of it is so compressed, that
it is impossible to distinguish the stars.’--‘Nov. 5, 1791, I viewed
Saturn with the 20 and 40 feet telescopes. _Twenty feet._ The fifth
satellite of Saturn is very small. The first, second, third, fourth and
fifth, and the new sixth satellites are in their calculated places.
_Forty feet._ I see the new sixth satellite much better with this
instrument than with the 20 feet. The fifth is also much larger here
than in the 20 feet, in which it was nearly the same size as a small
fixed star, but here it is considerably larger than that star.’
These examples, and many others of a similar kind, explain sufficiently
the nature and extent of that species of power that one telescope
possesses over another, in consequence of its enlarged aperture; but
the exact quantity of this power is in some degree uncertain. To
ascertain practically the illuminating power of telescopes, we must
try them with equal powers on such objects as the following,--the
small stars near the pole-star, and near Rigel and ε Bootis--the
division in the ring of Saturn--and distant objects in the twilight or
towards the evening. These objects are distinctly seen with a 5 feet
achromatic of 3-8/10 inches aperture, and an illuminating power of
144, while they are scarcely visible in a 3-1/2 feet with an aperture
of 2-3/4 inches, and an illuminating power of 72, supposing the same
magnifying power to be applied. The illuminating power of a telescope
is best estimated, in regard to land objects, when it is tried on
minute objects, and such as are badly lighted up; and the advantage
of a telescope with a large aperture will be most obvious, when it is
compared with another of inferior size in the close of the evening,
when looking at a printed bill composed of letters of various sizes. As
darkness comes on, the use of illuminating power becomes more evident.
In a 5 feet telescope some small letters will be legible, which are
hardly discernible in the 3-1/2 feet, and in the 2-1/2 feet are quite
undefinable, though the magnifying powers be equal. Sir W. Herschel
informs us, that in the year 1776, when he had erected a telescope of
20 feet focal length of the Newtonian construction, one of its effects
by trial was, that when towards evening, on account of darkness, the
natural eye could not penetrate far into space, the telescope possessed
that power sufficiently to show, by the dial of a distant church
steeple, what o’clock it was, notwithstanding the naked eye could no
longer see the steeple itself.
In order to convey an idea of the _numbers_ by which the degree of
space-penetrating power is expressed, and the general grounds on which
they rest, the following statements may be made. The depth to which the
naked eye can penetrate into the spaces of the heavens, is considered
as extending to the twelfth order of distances--in other words, it can
perceive a star at a distance 12 times farther than those luminaries,
such as Sirius, Arcturus or Capella, which, from their vivid light, we
presume to be nearest to us. It has been stated above, that Herschel
calculated his 10 feet telescope to have a space-penetrating power of
28.67, that is, it could enable us to descry a star 28 times farther
distant than the naked eye can reach. His 20 feet Newtonian was
considered as having a similar power of 61; his 25 feet, nearly 96, and
his 40 feet instrument, a power of 191.69. If each of these numbers
be multiplied by 12, the product will indicate how much farther these
telescopes will penetrate into space than the nearest range of the
fixed stars, such as those of the first magnitude. For instance, the
penetrating power of the 40 feet reflector being 191.69, this number
multiplied by 12, gives a product of 2,300, which shows, that were
there a series of two thousand three hundred stars extended in a line
beyond Sirius, Capella and similar stars--each star separated from the
one beyond it, by a space equal to the distance of Sirius from the
earth--they might be all seen through the 40 feet telescope. In short,
the penetrating power of telescopes is a circumstance which requires
to be particularly attended to in our observations of celestial
phenomena, and in many cases, is of more importance than _magnifying_
power. It is the effect produced by illuminating power that renders
telescopes, furnished with comparatively small magnifying powers, much
more efficient in observing comets and certain nebulæ and clusters of
stars, than when high powers are attempted. Every telescope may be so
adjusted, as to produce different space-penetrating powers. If we wish
to diminish such a power, we have only to contract the object-glass or
speculum, by placing circular rims, or apertures of different degrees
of breadth, across the mouth of the great tube of the instrument. But
we cannot increase this illuminating power beyond a certain extent,
which is limited by the diameter of the object-glass. When we wish
illuminating power beyond this limit, we must be furnished with an
object-glass or speculum of a larger size; and hence, the rapid advance
in price of instruments which have large apertures, and consequently
high illuminating powers. Mr. Tulley’s 3-1/2 feet achromatics of 2-3/4
inches aperture, sell at £26 5s. When the aperture is 3-1/4 inches, the
price is £42. When 3-3/4 inches, £68 5s. The following table contains a
statement of the ‘comparative lengths, apertures, illuminating powers,
and prices, of Achromatic Refractors, and Gregorian Reflectors,’
according to Dr. Kitchener.
---------------------------------------------------+
ACHROMATIC REFRACTORS. |
-----------+-----------+--------------+------------+
Length | Diameter | | |
and name | of | Illuminating | Price. |
they are | aperture. | power. | |
called by. | | | |
-----------+-----------+--------------+------------+
Feet. | In. Th. | | £ s. |
2 | 1. 6 | 25 | 4 4 |
2-1/2 | 2 | 40 | 12 12 |
3-1/2 | 2. 7 | 72 | 21 to 42 |
5 | 3. 8 | 144 | 105 to 150 |
7 | 5 | 250 | 250 |
7 | 6 | 360 | 360 |
----------------------------------------------------
| GREGORIAN &c. REFLECTORS.
+--------------+-----------+--------------+---------
| Length and | Diameter | |
| name they | of | Illuminating | Price.
| are | aperture. | power. |
| known by. | | |
+--------------+-----------+--------------+---------
| Feet. | In. Th. | | £ s.
| 1 | 2. 5 | 62 | 7 7
| 1-1/2 | 3 | 90 | 12 12
| 2 | 4. 5 | 202 | 20
| 3 | 5. 5 | 302 | 50
| 4 | 7 | 490 | 105
| 7 Newtonian | 7 | 490 | 126
| 5 Gregorian | 9 | 810 | 200
| 10 Newtonian | 10 | 1000 | 315
The illuminating powers stated in the above table are only comparative.
Fixing on the number 25 as the illuminating power of a 2 feet
telescope, 1-6/10 inch aperture, that of a 2-1/2 feet 2 inches inches
aperture, will be 40, of a 5 feet 3-8/10 inch aperture, 144, &c. If the
illuminating power of a Gregorian 1-1/2 foot, and 3 inches aperture, be
90, a 5 feet, with 9 inches aperture, will be 810, &c.
6. _On choosing Telescopes, and ascertaining their properties._
It is an object of considerable importance, to every astronomical
observer, that he should be enabled to form a judgment of the qualities
of his telescope, and of any instruments of this description which
he may intend to purchase. The following directions may perhaps be
useful to the reader in directing him in the choice of an achromatic
refracting telescope.
Supposing that an achromatic telescope of 3-1/2 feet focal length, and
3-1/4 inches aperture were offered for sale, and that it were required
to ascertain whether the object-glass, on which its excellence chiefly
depends--is a good one and duly adjusted;--some opinion may be formed
by laying the tube of the telescope in a horizontal position, on a
firm support, about the height of the eye,--and by placing a printed
card or a watch glass vertically, but in an inverted position, against
some wall or pillar, at 40 or 50 yards distant, so as to be exposed
to a clear sky. When the telescope is directed to this object, and
accurately adjusted to the eye--should the letters on the card, or the
strokes and dots on the watch-glass appear clearly and sharply defined,
without any mistiness or coloration, and if very small points appear
well defined--great hopes may be entertained that the glass will turn
out a good one. But a telescope may appear a good one, when viewing
common terrestrial objects, to eyes unaccustomed to discriminate
deviations from perfect vision, while it may turn out to be an
indifferent one, when directed to certain celestial objects. Instead
therefore of a printed card, fix a black board, or one half of a sheet
of black paper, in a vertical position at the same distance, and a
circular disk of white writing paper, about 1/4 of an inch in diameter,
on the centre of the black ground. Then having directed the telescope
to this object, and adjusted for the place of distinct vision, mark
with a black-lead pencil the sliding eye-tube, at the end of the main
tube, so that this position can always be known; and if this sliding
tube be gradually drawn out, or pushed in, while the eye beholds the
disk, it will gradually enlarge and lose its colour, till its edges
cease to be well-defined. Now, if the enlarged misty circle is observed
to be concentric with the disk itself, the object-glass is properly
centered, as it has reference to the tube; but if the misty circle goes
to one side of the disk, the cell of the object-glass is not at right
angles to the tube, and must have its screws removed and its holes
elongated, by a rattailed file, small enough to enter the holes. When
this has been done, the cell may be replaced, and the disk examined a
second time, and a slight stroke on one edge of the cell, by a wooden
mallet, will show by the alteration made in the position of the misty
portion of the disk, how the adjustment is to be effected, which is
known to be right when a motion in the sliding tube will make the
diluted disk enlarge in a circle concentric with the disk itself. When
the disk will enlarge so as to make a ring of diluted white light round
its circumference, as the sliding tube holding the eye-piece is pushed
in or drawn out, the cell may be finally fixed by the screws passing
through its elongated holes.
When the object-glass is thus adjusted, it may then be ascertained
whether the curves of the respective lenses composing the object-glass
are well-formed and suitable for each other. If a small motion of the
sliding tube of about 1/10th of an inch in a 3-1/2 feet telescope, from
the point of distinct vision, will dilute the light of the disk and
render the appearance confused, the figure of the object-glass is good;
particularly if the same effect will take place at equal distances
from the point of distinct vision, when the tube is alternately drawn
out and pushed in. A telescope that will admit of much motion in the
sliding tube without sensibly affecting the distinctness of vision,
will not define an object well at any point of adjustment, and must
be considered as having an imperfect object-glass, inasmuch as the
spherical aberration of the transmitted rays is not duly corrected. The
due adjustment of the convex lens, or lenses, to the concave one, will
be judged of by the absence of coloration round the enlarged disk, and
is a property distinct from the spherical aberration; the achromatism
depending on the relative focal distances of the convex and concave
lenses, is regulated by the relative dispersive powers of the pieces
of glass made use of; but the distinctness of vision depends on a good
figure of the computed curves that limit the focal distances. When an
object-glass is free from imperfection in both these respects, it may
be called a good glass for terrestrial purposes.
It still, however, remains to be determined how far such an
object-glass may be good for viewing a star or a planet, and can only
be known by actual observations on the heavenly bodies. When a good
telescope is directed to the moon or to Jupiter, the achromatism
may be judged of, by alternately pushing in, and drawing out the
eye-piece, from the place of distinct vision. In the former case, a
ring of purple will be formed round the edge; and in the latter, a
ring of light green, which is the central colour of the prismatic
spectrum; for these appearances show, that the extreme colours red
and violet are corrected. Again, if one part of a lens employed have
a different refractive power from another part of it, that is, if the
flint-glass particularly is not homogeneous, a star of the first and
even of the second magnitude will point out the natural defect by the
exhibition of an irradiation, or what is called _a wing_, at one side,
which no perfection of figure or of adjustment will banish, and the
greater the aperture the more liable is the evil to happen. Hence caps
with different apertures are usually supplied with large telescopes,
that the extreme parts of the glass may be cut off, in observations
requiring a round and well-defined image of the body observed.