608 OPT 
diately or remotely depends upon one narrow joint, are 
defective. 
In every ufeful (land the following properties ought to 
combine: Firft, the inftrument held by it fhould be kept 
firmly in its place, if of the refradting fort, fo that the 
image may have no vibratory motion unfavourable to dif¬ 
tindt vifion, occalioned by an unfteady pofition of the ob- 
jedl-glafs; but, if it be of the refledting kind, then it 
fhould be fo fufpended, that tremulous motions ariiing 
from compadtnefs of the materials, fuch as eafily tranf- 
niit vibratory impulfes, may be avoided. Secondly, a 
motion in azimuth, and another in altitude, are itidif- 
penfable and, if the inftrument be bulky, or have great 
power, in each of thefe refpedts there Ihould be both a 
quick and a flow motion, the former to fave time, and 
the latter for the fake of accuracy. Thirdly, when the 
inftrument is pointed to the required objedt, it fhould 
not be liable to be eafily moved by any accidental touch 
of the obferver’s hand or body ; unlefs it is managed in 
a ftate of fufpenfion, as is the cafe with the larger reflec¬ 
tors. Fourthly, the parts of the ftand fhould be ftrong 
enough to bear the fuperincumbent weight, and not lia¬ 
ble to get out of repair; and fifthly, its pofition fhould 
be in a fituation not eafily fliaken, or moved by the ob¬ 
ferver’s weight, or that of a by-ftander. When thefe pro¬ 
perties are attended to, the exadt fhape and external 
appearance becomes matter offecondary confideration, and 
each artift may-purfue his own fchemein the conftrudtion. 
One of the bell (lands for a thirty-inch refrador, by 
■which we always mean an achromatic refradtor, is that 
exhibited at fig. 4. in which AB is a tube of brafs, 
mounted on the tripod ftand of the fame metal CD, and 
fixed by means of the fcrews Q and R. In the common 
conftrudtion, the horizontal motion is at C, at the top of 
the ftem or cylinder, and the fyftem of tubes F, E, P, is 
not applied, fo that there is neither the flow adjuftable 
motions, nor is the inftrument fteady in any given pofi¬ 
tion ; but here the horizontal motion is at D, at the lower 
extremity of the cylinder, where there is a long bearing 
for the pivot, with a tightening (crew underneath the 
jundtion of the feet, and a clamp S to fix the inftrument 
in any given diredtion. Alfo the handle in fig. 5. with a 
Hooke’s joint, taking the fquared axis of the fcrew at D, 
gives the flow motion in azimuth, while the Aiding and 
adjuftable tubes F, E, P, keep the angle of elevation un¬ 
altered. Thefe tubes turn on a joint at P; and, when a 
due elevation is given, by the quick motion occafioned by 
the freedom of Hiding one within another, a clamping- 
fcrew at E fixes- them, and the flow motion produced by 
the fcrew F finifhes the obfervation in altitude, as the 
handle in fig. 5 does in azimuth; and both flow mo¬ 
tions can be managed, one with each hand, at the fame 
time. In the prefent reprefentation, the celeftial eye¬ 
piece H is fcrewed into its fmall tube, which bears a con¬ 
cealed rack, that is ad'ted on by a pinion on the axis of 
the thumb-piece G, which may be made more or lefs 
tight by a fcrew in the middle of its plane, and which 
adjulls the eye-pieces for diftindt vifion. There may be 
any number of various celeftial eye-pieces, but two or 
three are as many as are ufually delivered with an inftru¬ 
ment of this fize. The objedt-glafs fcrews into the 
mouth of the tube at B, and is fo fixed by trial, that the 
mofl diftindt view of an objedl is had when the fcrew is 
carried home, in which fituation the receiving-focket is 
fixed by the maker; fo that unfcrewing the objedt-glafs 
at any time does not injure the inftrument. The centre 
of motion in altitude is at a joint above C, and the ftea- 
dinefs of the tube AB will depend on the diftance of this 
joint from the fyftem of concentric tubes F, E, P. When 
the cylindrical piece beyond the joint of the loweft tube at 
P is withdrawn from its hole in a cock attached to the 
vertical cylinder, the tubes will pack into one another, 
and the cylindrical end-piece will enter the hole of the 
cock T, under the main tube, and remain out of the 
■way of injury, parallel to this tube. The tube IK, in 
I c s. 
fig. 6, fcrews at the end I into the fame place that the 
celeftial eye-piece H now occupies, and is called the ter - 
rejirial tube , or terreftrial eye-piece, becaufe objedls are 
feen in their diredl pofition through it, which, through 
the celeftial eye-piece, are feen inverted. Near the end I, 
a pair of glaffes, called the field-glajjes, are fcrewed, and 
the end K contains the pair of glaffes which is denomi¬ 
nated the eye-piece. We have already fhown that thefe 
two pairs of glaffes conftitute an achromatic eye-piece, at 
the fame time that they eredt the inverted image formed 
by the objedt-glafs in the fmall tube between H and A ; 
and, when this image is confidered as a real objedl, then 
the terreftrial tube is a compound microfcope of the beft 
conftrudtion. The tube L, in fig. 7. is an open tub®, 
which is fometimes made, by particular defire, to receive 
at its end L the eye-piece, now fcrewed into the tube IK 
at K ; and then, as the empty tube L Aides in the tube 
IK, the diftance between the pair of field-glaffes and pair 
of eye-glaffes may be varied at pleafure ; and, as the mag¬ 
nifying power of the compound microfcope varies directly 
with this diftance, it is evident that the power of the te- 
lefcope thus conftrudted will vary in like manner. The 
Undent will readily comprehend the advantages and de- 
fedts of this apparatus. It is very portable, becaufe the 
telefcope is eafily difengaged from it, and the legs and 
flay fold up. If the joint C were immediately under B, 
it would be much freer from all tremor in the vertical 
plane. But nothing can hinder other tremors arifing 
from the long pillar and the three fpringy legs. Thefe 
communicate all external agitations with great vigour. 
The inftrument fhould be fet on a ltone pedeltal, or, what 
is better, a calk filled with wet fand. This pedeltal, which 
necelfity perhaps fuggefted to our fcientific navigators, is 
the beft that can be imagined. 
Fig. 8 is the ftand ufually given to rejleding telefcopes. 
AB is the main tube of a refledting telefcope of moderate 
fize, which may be either of the Gregorian or Cafi'e- 
grainian kind, (for their external appearance and mode of 
uling are the lame,) mounted on the. tripod GF ; at A is 
the open aperture, and a little fnort of it, within, is the 
fmall fpeculum, drawn in or out by the fcrew C, which is 
connected, with a longitudinal bar of metal, into which 
the heel-piece of the ftem of the fmall fpeculum Hides, fo 
as to be taken away, or put in, at pleafure; within the in¬ 
terior end B of the large tube is the perforated large fpe¬ 
culum, always concave, and of a proper figure to fuit the 
face of the fmall fpeculum. This large fpeculum is put 
in with fome liberty, as is alfo the fmall one in a box con¬ 
taining a fpiral fpring adding againft it to prevent tremors; 
B is the eye-piece of the Huygenian kind, of which there 
are ufually two or three, according to the fize of the in¬ 
ftrument. The femicircle D is racked ; and tjie handle 
H turns the fcrew that gives elevation, while the handle 
I gives the horizontal motion, by its fcrew driving the 
racked horizontal plate E ; both which are clearly feen in 
the figure. The verticaL motion takes place at the centre 
of gravity of the tube at the top of the frame between 
E and D, and the horizontal motion is from an axis in 
the centre of the racked plate E, which axis has a long 
hole, bearing down a tube to G, under the jundtion of 
the three legs. The three-barred bracing-piece F has a 
joint at each leg, and alfo at the circular brafs plate in 
the centre, fo that a little force applied under this plate 
will raife it, and allow the legs to come together into 
contadl, as well as the three, arms that keep the legs open 
when the ftand is ufed. This mounting is very portable, 
and tolerably fteady, particularly when the tube is fhort, 
and is every way convenient for ufe, except that both 
the vertical and horizontal motions are flow motions; the 
former of which is tedious when a great change of alti¬ 
tude is wanted in a given time, but the latter is in fome 
meafure remedied by the portability of the ftand, which 
may be eafily turned all together, to face any particular 
objedl. We have feldom (fays the editor of the Encyclo¬ 
pedia Britannica) feen diftindtly through telefcopes 
mounted 
