The Practical Astronomer: Comprising illustrations of light and colours--practical descriptions of all kinds of telescopes--the use of the equatorial-transit--circular, and other astronomical instruments, a particular account of the Earl of Rosse's large telescopes, and other topics connected with astronomyDick, Thomas
Science
The Practical Astronomer: Comprising illustrations of light and colours--practical descriptions of all kinds of telescopes--the use of the equatorial-transit--circular, and other astronomical instruments, a particular account of the Earl of Rosse's large telescopes, and other topics connected with astronomy
Dick, Thomas
Astronomical instruments; Astronomy; Telescopes
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
Public-domain text, read in full here on John Shaqi.
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