The speculum is 5-1/4 inch diameter, and carefully figured, and is
recommended as a very useful instrument. The size and power of this
telescope has been adopted as that most likely to meet the means and
requirements of a large number of amateurs. Many prefer to commence
astronomical observations with “_something inexpensive_,” and are
led to begin with the popular and well advertised 3 inch refractor
at £5. These, except in a few chance cases, are sure to prove
disappointing, and are perhaps the cause of their giving up any
further attempts to follow up the subject of astronomy, which may
otherwise be so pleasantly and profitably pursued with a reliable
instrument.
It is well known that a less aperture than 4-1/2 inches is
insufficient to give the observer a satisfactory idea of the varied
and most interesting details of the planets, and the ever-changing
outline and tone of the belts and markings of Jupiter, Saturn, and
Mars. Large apertures bring out _details_ when the smaller ones can
only show a _general outline_. The former also delineates more with
a lower power, providing the focus is of proper length.
The planets and nebulæ cannot be seen to advantage without aperture
and focal length. The field of view is then flat, the object is
properly illuminated, with sharp and crisp definition, and is also
_much_ less subject to annoyance from tremour, through the necessity
for constant adjustments in the fields, &c., as is the case with
small apertures, for the object is magnified more in proportion. A
certain magnifying power is necessary in order that the object may
be sufficiently large to scrutinize; and this, whether the aperture
is large or small, must be from about 150 to about 300 times for the
planets. But 150 on a 3 inch is a high power for the quantity of
light obtained, to say nothing of the separate consideration and
advantage of long focus. Aperture is a quality or function of the
telescope considered separately from light or focus. For instance,
suppose a 5 inch speculum is so thinly silvered that it gives the
exact degree of light as a 2-1/2 inch refractor, the defining and
separating power of the 5 inch aperture would be very superior to
the 2-1/2 inch.
The “Popular Reflector,” with 5-1/4 inch speculum of 5, 5-1/2, to 6
feet focus, will be found very suitable, and if its illuminating
power is not greater than a 4-1/2 inch refractor, its defining and
dividing powers are superior. By choosing the above focal lengths,
according to circumstances, the observer can reach the eye-tube,
while standing erect for objects in the zenith, and it is a great
mistake to suppose that the shorter the tube the more handy it
becomes. These foci will not require a “Barlow” lens to flatten the
field. The “Barlow” is very useful for short foci when the aperture
is considerable, as it improves the imperfect correction for
spherical error, but this is much better corrected in the mirror
itself than by a “Barlow” lens, which cannot be used without more
than one disadvantage. There must, by its insertion, be some loss of
light, which can ill be spared with small instruments, and when used
to obtain magnifying power there is some disturbance of colour, and
this subtracts from the beauty and purity of the definition of a
reflector. There is _nothing_ equal to a good eye-piece to obtain
_power_, and _flatness_ of field by the _curve_ of the speculum.
With the “Popular Reflector” and an outlay of a few shillings on
some popular books, such as the excellent work “Celestial Objects,”
by the Rev. T. W. Webb, and some first-class publications by Mr. R.
A. Proctor, especially his smaller star atlas, &c., the amateur can
compare the work he is then capable of doing with a large and
expensive refractor which might be beyond his reach.