70 
THE YOUNG SCIENTIST. 
row arms from the sides, and leaving an 
open ring of light all round, may be tried. 
But for comets or nebulae, it will be best 
to restore the original aperture, as with 
faint and ill-deflned objects light is more 
essential than distinctness. The center- 
ing of a reflector is more liable to derange- 
ment than that of an achromatic; it is, 
however, easily rectified by the cautious 
use of the screws which are provided for 
the purpose. When in correct adjust- j 
ment, the eye-piece being removed, a darlc 
spot will be seen in the centre of the small 
mirror, which is the image of that mirror 
reflected by the large speculum ; in pro- 
portion as it deviates from a central posi- 
tion, the adjustment is incomplete, and 
the performance defective. 
The deflnition of reflectors may often be 
greatly improved by the use of a tube per- 
forated with large and numerous open- 
ings ; it is also desirable, where practica- 
ble, to interpose between the observer and 
the instrument, in cold weather, a move- 
able screen of felt or some non-conducting 
material. 
A good stand is essential ; if unsteady, 
it will spoil the most distinct i^erform- 
ance; if awkward, it will annoy the ob- 
server ; if limited in range, it may disap- 
point him at some interesting juncture. 
It may be well left to a respectable op- 
tician ; but where expense is a serious 
consideration, a little mechanical inge- 
nuity and knowledge of such contrivances 
will devise one which will answer suf- 
ficiently. The old arrangement, with a 
vertical and horizontal, or " altitude and 
azimuth" motion, is simple and manage- 
able; the equatorial form, which makes 
the telescope revolve on an axis parallel 
to that of the earth, has great advantages, 
in following the object by a single motion, 
and where the expense of divided circles 
and spirit-levels is admissible, in finding 
planets and bright stars by day, and iden- 
tifying minute objects by night ; but, to 
do its work, it must be placed accurately 
in the meridian, and out of that posi- 
tion has little superiority. The tube, too, 
must rotate in a cradle, or the ocular will 
assume very awkward inclinations. In 
any case, if the stand is to be movable, 
let it be strong enough for steadiness 
without being too heavy for portability.* 
A sidereal clock is often considered a 
necessary adjunct to an equatorial mount- 
ing, in order to find objects invisible to 
the naked eye. But it may be dispensed 
with by the following method of " dif- 
ferentiation " in all cases, excepting dur- 
ing the brief season of twilight, when 
neither sun nor stars can be employed. 
Write down the difference of Eight As- 
j cension (taking particular notice whether 
additive or subtractive) between the re- 
quired and some known object— the sun 
by day, a neighboring bright star by 
night. Seek the known object by the 
finder, and place it in the centre of your 
largest field ; clamp the R. A. circle ; set 
the telescope to the declination of the ob- 
ject sought, and clamp it there ; un clamp 
in R. A. and move the telescope E. or W., 
as the case requires, to the value of the 
ascertained difference in R. A., and the 
object will be found^in the field, somewhat 
W. of the centre, by a distance dependent 
on the duration of the process. 
An observatory is by no means essen- 
tial, but it would be difflcult to over- 
estimate its advantage in i^oint of comfort 
as well as economy of time. It used to be 
an expensive luxury; but a very simple 
and cheap "telescope-house," combining 
shelter with open-air freedom, to the great 
merit of wdiich I can bear full testimony, 
has been devised by the Rev. E. L. Ber- 
thon, and is described in the English 
Mechanic, Oct. 13 and 20, 1871. The Rev. 
W. Conybeare Bruce has also handled the 
subject very ably in the same publication, 
Eeb. 6 and 27, and April 2, 1880. 
We will close this section with the en- 
couraging words of the Council of the 
Royal Astronomical Society, in their Re- 
port for 1828 : " Every one who possesses 
an instrument, whose claims rise even 
not above a humble mediocrity, has it in j 
his power to chalk out for himself a ! 
* A very cheap eauatorial stand is described in 
1 Astron. Register, xiv. 35. Franks observes that a : 
common equatorial mounting may be made very i 
efficient at a trifling expense by the addition of ! 
plain metal circles, on which slips of paper gradu- 
ated with pen and ink are fastened by glue dis- 
solved in strong acetic acid, and afterwards sized 
and varnished. A good pillar-stand may be 
made by letting a 4-inch iron pipe deep into the 
ground, in which a small table with a long foot 
revolves. I 
