William F. Denning.

Telescopic Work for Starlight Evenings online

. (page 3 of 32)
Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 3 of 32)
Font size
QR-code for this ebook


such excellent character as to compensate in a measure for feeble
illumination.

In discussing this question it will be advisable to glance at the
performances of certain instruments of considerable size.

The introduction of really large glasses dates from a century ago,
when Sir W. Herschel mounted his reflector, 4 feet in aperture, at
Slough. He discovered two of the inner satellites of Saturn very soon
after it was completed; but apart from this the instrument seems to
have achieved little. Herschel remarked that on August 28, 1789, when
he brought the great instrument to the parallel of Saturn, he saw
the spots upon the planet better than he had ever seen them before.
The night was probably an exceptionally good one, for we do not find
this praise reiterated. Indeed, Herschel appears to have practically
discarded his large instrument for others of less size. He found that
with his small specula of 7-ft. focus and 6·3-in. aperture he had
“light sufficient to see the belts of Saturn completely well, and
that here the maximum of distinctness might be much easier obtained
than where large apertures are concerned.” Even in his sweeps for
nebulæ he employed a speculum of 20-ft. focus and 18½-in. aperture in
preference to his 4-ft. instrument, though on objects of this nature
light-grasping power is essentially necessary. The labour and loss of
time involved in controlling the large telescope probably led to its
being laid aside for more ready means, though Herschel was not the man
to spare trouble when an object was to be gained. His life was spent
in gleaning new facts from the sky; and had the 4-foot served his
purpose better than smaller instruments, no trifling obstacle would
have deterred him from its constant employment. But his aim was to
accomplish as much as possible in every available hour when the stars
were shining, and experience doubtless taught him to rely chiefly upon
his smaller appliances as being the most serviceable. The Le Mairean
form, or “Front view,” which Herschel adopted for the large instrument
may quite possibly have been in some degree responsible for its bad
definition.

[Illustration: Fig. 10.

Lord Rosse’s 6-foot Reflecting-Telescope.]

Lord Rosse’s 6-ft. reflector has now been used for nearly half a
century, and its results ought to furnish us with good evidence as
to the value of such instruments. It has done important work on the
nebulæ, especially in the re-observation of the objects in Sir J.
Herschel’s Catalogues of 1833 and 1864. To this instrument is due
the discovery of spiral nebulæ; and perhaps this achievement is its
best. But when we reflect on the length of its service, we are led
to wonder that so little has been accomplished. For thirty years the
satellites of Mars eluded its grasp, and then fell a prize to one
of the large American telescopes. The bright planets[5] have been
sometimes submitted to its powers, and careful drawings executed by
good observers; but they show no extent of detail beyond what may be
discerned in a small telescope. This does not necessarily impugn the
figure of the large speculum, the performance of which is entirely
dependent upon the condition of the air. The late Dr. Robinson, of
Armagh, who had the direction of the instrument for sometime, wrote
in 1871:—“A stream of heated air passing before the telescope, the
agitation and hygrometric state of the atmosphere, and any differences
of temperature between the speculum and the air in the tube are all
capable of injuring or even destroying definition, though the speculum
were absolutely perfect. The effect of these disturbances is, in
reflectors, as the cube of their apertures; and hence there are few
hours in the year when the 6-foot can display its full powers.” Another
of the regular observers, Mr. G. J. Stoney, wrote in 1878:—“The usual
appearance [of the double star γ^2 Andromedæ] with the best mirrors was
a single bright mass of blue light some seconds in diameter and boiling
violently.” On the best nights, however, “the disturbance of the air
would seem now and then suddenly to cease for perhaps half a second,
and the star would then instantly become two very minute round specks
of white light, with an interval between which, from recollection, I
would estimate as equal to the diameter of either of them, or perhaps
slightly less. The instrument would have furnished this appearance
uninterruptedly if the state of the air had permitted.” The present
observer in charge, Dr. Boeddicker, wrote the author in 1889:—“There
can be no doubt that on favourable nights the definition of the 6-foot
is equal to that of any instrument, as is fully shown by Dr. Copeland’s
drawings of Jupiter published in the ‘Monthly Notices’ for March 1874.
It appears to me, however, that the advantage in going from the 3-foot
to the 6-foot is not so great in the case of planets as in the case
of nebulæ; yet, as to the Moon, the detail revealed by the 6-foot on
a first-class night is simply astounding. The large telescope is a
Newtonian mounted on a universal joint. For the outlying portions of
the great drawing of the Orion nebula it was used as a Herschelian. As
to powers profitably to be used, I find no advantage in going beyond
600; yet formerly on short occasions (not longer than perhaps 1 hour
a night) very much higher powers (over 1000) have been successfully
employed by my predecessors.”

Mr. Lassell’s 4-foot reflector was taken to Malta, and while there its
owner, assisted by Mr. Marth, discovered a large number of nebulæ with
it, but it appears to have done nothing else. His 2-foot reflector,
which he had employed in previous years, seems to have been his most
effective instrument; for with this he discovered Ariel and Umbriel,
the two inner satellites of Uranus, Hyperion, the faintest satellite
of Saturn, and the only known satellite of Neptune. He also was one
of the first to distinguish the crape ring of Saturn. Mr. Lassell had
many years of experience in the use of large reflectors; and in 1871 he
wrote:—“There are formidable and, I fear, insurmountable difficulties
attending the construction of telescopes of large size.... These are,
primarily, the errors and disturbances of the atmosphere and the
flexure of the object-glasses or specula. The visible errors of the
atmosphere are, I believe, generally in proportion to the aperture
of the telescope.... Up to the size [referring to an 8-in. O.-G.] in
question, seasons of tranquil sky may be found when its errors are
scarcely appreciable; but when we go much beyond this limit (say to 2
feet and upwards), both these difficulties become truly formidable. It
is true that the defect of flexure may be in some degree eliminated,
but that of atmospheric disturbance is quite unassailable. These
circumstances will always make large telescopes _proportionately_ less
powerful than smaller ones; but notwithstanding these disadvantages
they will, on some heavenly objects, reveal more than any small ones
can.” Mr. Lassell’s last sentence refers to “delineations of the forms
of the fainter nebulæ,” to “seeing the inner satellites of Uranus, the
satellite of Neptune, and the seventh satellite of Saturn.” He mentions
that, when at Malta, he “saw, in the 2-foot equatoreal, with a power of
1027, the two components of γ^2 Andromedæ distinctly separated to the
distance of a neat diameter of the smaller one. Now, no telescope of
anything like 8-inches diameter could exhibit the star in this style.”

The large Cooke refractor of 24·8-inches aperture, which has been
mounted for about twenty years at Gateshead, has a singularly barren
record. Its atmospheric surroundings appear to have rendered it
impotent. The owner of this fine and costly instrument wrote the author
in 1885:—“Atmosphere has an immense deal to do with definition. I have
only had one fine night since 1870! I then saw what I have never seen
since.”

The Melbourne reflector of 4-feet aperture performed very indifferently
for some years, and little work was accomplished with it. Latterly its
performance has been more satisfactory; excellent photographs of the
Moon have been taken, and it has been much employed in observations
of nebulæ. The speculum having recently become tarnished, it has been
dismounted for the purpose of being repolished.

The silver-on-glass reflector of 47·2-in. diameter, at the Paris
Observatory, was used for some years by M. Wolf, who has also had the
control of smaller telescopes. He was in a favourable position to judge
of their relative effectiveness. In a lecture delivered at the Sardonne
on March 6, 1886, he said:—“During the years I have observed with the
great Parisian telescope I have found but one solitary night when the
mirror was perfect.” Further on, he adds:—“I have observed a great
deal with the two instruments [both reflectors] of 15·7 inches and
47·2 inches. I have rarely found any advantage in using the larger one
when the object was sufficiently luminous.” M. Wolf also avers that a
refractor of 15 inches or reflector of 15·7 inches will show everything
in the heavens that can be discovered by instruments of very large
aperture. He always found a telescope of 15·7-inch aperture surpass one
of 7·9 inches, but expresses himself confidently that beyond about 15
inches increased aperture is no gain.

The Washington refractor of 25·8 inches effected a splendid success
in Prof. Hall’s hands in 1877, when it revealed the two satellites of
Mars. But immediately afterwards these minute bodies were shown in
much smaller instruments; whence it became obvious that their original
discovery was not entirely due to the grasp of the 25·8-inch telescope,
but in a measure to the astuteness displayed by Prof. Hall in the
search. A good observer had been associated with a good telescope; and
an inviting research having been undertaken, it produced the natural
result—an important success. The same instrument, in the same hands,
enabled the rotation-period of Saturn to be accurately determined by
means of a white spot visible in December 1876 on the disk of the
planet, and which was subsequently seen by other observers with smaller
glasses. Good work in other directions has also been accomplished
at Washington, especially in observations of double stars and faint
satellites. But notwithstanding these excellent performances, Prof.
Hall expressed himself in rather disparaging terms of his appliances,
saying “the large telescope does not show enough detail.” He gave
a more favourable report in 1888; for we find it stated that “the
objective retains its figure and polish well. By comparison with
several other objectives which Prof. Hall has had an opportunity of
seeing during recent years, he finds that the glass is an excellent
one.”

Prof. Young, who has charge of the 23-inch refractor at Princeton, has
also commented on the subject of the definition of large telescopes. He
says:—“The greater susceptibility of large instruments to atmospheric
disturbances is most sadly true; and yet, on the whole, I find also
true what Mr. Clark told me would be the case on first mounting
our 23-inch instrument, that _I can almost always see with the
23-inch everything I see with the 9½-inch under the same atmospheric
conditions, and see it better_,—if the seeing is bad only a little
better, if good immensely better.” Prof. Young also mentioned that
a power of 1200 on the 23-inch “worked perfectly on Jupiter on two
different evenings in the spring of 1885 in bringing out fine details
relating to the red spot and showing the true forms of certain white
dots on the S. polar belt.”

The 26-inch refractor at the Leander McCormick Observatory, U. S.
A., is successfully engaged in observations of nebulæ, and many new
objects of this character have been found. It does not appear that the
telescope is much used for other purposes; so that we can attach no
significance to the fact that important discoveries have not been made
with it in other departments.

The great Vienna refractor of 27-inches aperture “does not seem to
accomplish quite what was expected of it,” according to Mr. Sawerthal,
who recently visited the Observatory at Währing, Vienna. The Director,
Dr. Weiss, states in his last report that “the 27-inch Grubb refractor
has only been occasionally used, when the objects were too faint for
the handier instruments.”

The still larger telescopes erected at the Observatories at Pulkowa and
Nice have so recently come into employment that it would be premature
to judge of their performance. In the Annual Report from Pulkowa
(1887) it is stated that Dr. H. Struve was using the 30-inch refractor
“in measuring those of Burnham’s double stars which are only seldom
measurable with the ‘old 15-inch,’ together with other stars of which
measures are scarce. He made 460 measures in eight or nine months, as
well as 166 micro metric observations of the fainter satellites of
Saturn and 15 of that of Neptune.” At Nice the 30-inch refractor was
employed by M. Perrotin in physical observations of Mars in May and
June 1888. The canal-shaped markings of Schiaparelli were confirmed,
and some of them were traced “from the ocean of the southern hemisphere
right across both continents and seas up to the north polar ice-cap.”
The 30-inch also showed some remarkable changes in the markings;
but these were not confirmed at other observatories. The telescope
evidently revealed a considerable amount of detail on this planet;
whence we may infer that its defining power is highly satisfactory.

The great Lick refractor, which appears to have been “first directed to
the heavens from its permanent home on Mount Hamilton on the evening of
January 3, 1888,” has been found ample work by the zealous astronomers
who have it in charge. Prof. Holden, in speaking of it, says:—“It needs
peculiar conditions, but when all the conditions are favourable its
performance is superb.” Mr. Keeler, one of the observers, writes that,
on January 7, 1888, when Saturn was examined, “he not only shone with
the brilliancy due to the great size of the objective, but the minutest
details of his surface were visible with wonderful distinctness. The
outlines of the rings were very sharply defined with a power of 1000.”
Mr. Keeler adds:—“According to my experience, there is a direct gain in
power with increase of aperture. The 12-inch equatoreal brings to view
objects entirely beyond the reach of the 6½-inch telescope, and details
almost beyond perception with the 12-inch are visible at a glance with
the 36-inch equatoreal. The great telescope is equal in defining power
to the smaller ones.” This is no small praise, and it must have been
extremely gratifying, not only to those who were immediately associated
with the construction of the telescope, but to astronomers everywhere
who were hoping to hear a satisfactory report. In its practical results
this instrument has not yet, it is true, given us a discovery of any
magnitude. It has disclosed several very small stars in the trapezium
of the Orion nebula, some difficult double stars have been found and
measured, and some interesting work has been done on the planets and
nebulæ. Physical details have been observed in the ring nebula, between
β and γ Lyræ, which no other telescope has ever reached before.

Mr. Common’s 5-foot reflector has been employed on several objects.
In the spring of 1889 Uranus was frequently observed with it, and
several minute points of light, suspected to be new satellites, were
picked up. Evidence was obtained of a new satellite between Titania
and Umbriel; but bad weather and haze, combined with the low altitude
of Uranus, interfered with the complete success of the observations.
“With only moderate powers, Uranus does not show a perfectly sharp
disk. No markings are visible on it, and nothing like a ring has been
seen round it.” Mr. Common, in a letter to the writer, dated November
9, 1889, says:—“The 5-foot has only been tried in an unfinished state
as yet, the mirror not being quite finished when put into the tube
last year. This was in order to gain experience and save the season.
It performed much better than I had hoped, and is greatly superior to
the 3-foot. I took some very fine photographs with it last year. It
has been refigured, or rather completed, this summer, and has just
been resilvered.” From this it is evident that Mr. Common’s large
instrument has not yet been fully tested; but it clearly gives promise
of successful results, and encourages the hope that it will exert an
influence on the progress of astronomy. Owing to the highly reflective
quality of silvered glass, the 5-foot speculum has a far greater
command of light (space-penetrating power) than the great objective
mounted at the Lick Observatory. Mr. Common’s mirror may therefore be
expected to grasp nebulæ, stars, satellites, and comets which are of
the last degree of faintness and quite invisible in the Lick refractor.
But we must not forget that the latter instrument is certainly placed
in a better atmosphere, and that its action is not therefore arrested
in nearly the same degree by haze and undulations of the air. With
equal conditions, the great reflector at Ealing would probably far
surpass the large refractor we have referred to, the latter having less
than one third of the light-grasping power of the former.

This rapid sketch of the performances of some of our finest telescopes
must suffice for the present in assisting us to estimate their value as
instruments of discovery. And it must be admitted that, on the whole,
these appliances have been disappointing. The record of their successes
is by no means an extended one, and in some individual cases absolute
failure is unmistakable. We must judge of large glasses by their
revelations; their capacity must be estimated by results. We often meet
with glowing descriptions of colossal telescopes: their advantages
are specified and their performances extolled to such a degree that
expectation is raised to the highest pitch. But it is not always that
such praise is justified by facts. The fruit of their employment is
rarely prolific to the extent anticipated, because the observers have
been defeated in their efforts by impediments which inseparably attend
the use of such huge constructions.

Our atmosphere is always in a state of unrest. Its condition is subject
to many variations. Heat, radiated or evolved from terrestrial objects,
rises in waves and floats along with the wind. These vapours exercise a
property of refraction, with the result that, as they pass in front of
celestial objects, the latter at once become subject to a rapid series
of contortions in detail. Their outlines appear tremulous, and all the
features are involved in a rippling effect that seriously compromises
the definition. Delicate markings are quite effaced on a disk which
is thus in a state of ebullition; and on such occasions observers are
rarely able to attain their ends. Telescopic work is, in fact, best
deferred until a time when the air has become more tranquil. In large
instruments these disturbances are very troublesome, as they increase
proportionately with aperture. They are so pronounced and so persistent
as to practically annul the advantage of considerable light-grasping
power; for unless the images are fairly well defined, mere brightness
counts for nothing. Reflectors are peculiarly susceptible to this
obstacle; moreover, the open tube, the fact that rays from an object
pass twice through its length, and that a certain amount of heat
radiated from the observer must travel across the mouth of the tube
all serve to impair the definition. A speculum, to act well, must be
of coincident temperature in every part. This is not always the case,
owing to the variableness of the weather or to unequal exposure of the
speculum. Large refractors, though decidedly less liable to atmospheric
influences, are yet so much at the mercy of them that one of the first
and most important things discussed in regard to a new instrument is
that of a desirable site for it.

The great weight of large objectives and specula tends to endanger the
perfect consistency and durableness of their figure, and imposes a
severe strain upon their cellular mounting. The glasses must obviously
assume a variety of bearings during active employment. This introduces
a possible cause of defective performance; for in some instances
definition has been found unequal, according to the position of the
glass. Specula are very likely to be affected in this manner, as they
are loosely deposited in their cells to allow of expansion, and the
adjustment is easily deranged. The slightest flaw in the mounting of
objectives immediately makes itself apparent in faulty images. Special
precautions are of course taken to prevent flexure and other errors of
the kind alluded to, and modern adaptations may be said to have nearly
eliminated them; but there is always a little outstanding danger, from
the ease with which glasses may be distorted or their adjustment become
unsettled.

Another difficulty formerly urged against telescopes of great size
was the trouble of managing them; but this objection can scarcely
be applied to the fine instruments of the present day, which are
so contrived as to be nearly as tractable as small ones. A century
ago, glass of the requisite purity for large objectives could not be
obtained; but this difficulty appears also to have quite disappeared.
And the process of figuring lenses of considerable diameter is now
effected with the same confidence and success as that of greatly
inferior sizes.

Let us now turn for a moment to the consideration of small instruments,
premising that in this category are included all those up to about
12-inches aperture. Modern advances have quite altered our ideas as to
what may be regarded as large and small telescopes. Sixty-five years
ago the Dorpat refractor, with a 9½-inch objective by Fraunhofer, was
considered a prodigy of its class; now it occupies a very minor place
relatively to the 30-inch and 36-inch objectives at Nice, Pulkowa, and
Mount Hamilton.

Prof. Hall remarked, in 1885:—“There is too much scepticism on the part
of those who are observing with large instruments in regard to what
can be seen with small ones.” This is undoubtedly true; but a mere
prejudice or opinion of this sort cannot affect the question we are
discussing, as it is one essentially relying upon facts.

Small instruments have done a vast amount of useful work in every
field of astronomical observation. Even in the realm of nebulæ, which,
more than any other, requires great penetrating power, D’Arrest showed
what could be effected with small aperture. Burnham, with only a
6-inch refractor, has equally distinguished himself in another branch;
for he has discovered more double stars than any previous observer.
Dawes was one of the most successful amateurs of his day, though his
instrumental means never exceeded an 8-inch glass. But we need not
particularize further. It will be best to get a general result from
the collective evidence of past years. We find that nearly all the
comets, planetoids, double stars, &c. owe their first detection to
comparatively small instruments. Our knowledge of sun-spots, lunar and
planetary features is also very largely derived from similar sources.
There is no department but what is indebted more or less to the
services of small telescopes: the good work they have done is due to
their excellent defining powers and to the facility with which they may
be used.

[Illustration: Fig. 11.

Refracting-Telescope, by Browning.]

We have already said that the record of discoveries made with really
large instruments is limited; but it should also be remarked that until
quite recently the number of such instruments has been very small.
And not always, perhaps, have the best men had the control of them.
Virtually the observer himself constitutes the most important part of
his telescope: it is useless having a glass of great capacity at one
end of a tube, and a man of small capacity at the other. Two different
observers essentially alter the character of an instrument, according
to their individual skill in utilizing its powers.

Large telescopes are invariably constructed for the special purpose of
discovering unknown orbs and gleaning new facts from the firmament.
But in attempting to carry out this design, obstacles of a grave



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 3 of 32)