OBL 
elyects the brightest and most distinct, and 
which bears the greatest aperture, and the 
most convex and concave eye-glasses, with- 
out colouring or haziness. A circular ob- 
ject-glass is said to be truly centered when 
the centre of its circumference falls ex- 
actly in the axis of the glass; and to be 
ill centered when it falls out of the axis. 1 o 
prove whether object-glasses be well cen- 
tered, hold tlie glass at a due distance from 
the eye, and observe the two reflected 
images of a candle, varying tlie distance till 
the two images unite, which is the true 
centre point: then if this fall in the middle, 
or central point of the glass, it is known to 
be truly centered. As object-glasses are 
commonly included in cells that screw upon 
the end of the tube of a telescope, it may 
be pro ved whether they be well centered by 
fixing the tube and observing, while the 
cell is unscrewed, whether the cross-hairs 
keep fixed upon the same lines of an object 
seen through the telescope. 
OBJECTIVE line, in perspective, is any 
line drawn on the geometrical plane, whose 
representation is sought for in a drairght or 
picture : and the objective plane is any plane 
situated in the horizontal plane, the repre- 
sentation of which is required. See Per- 
spective. 
OBLATE, flattened, or shortened, as an 
oblate spheroid, havuig its axis shorter than 
its middle diameter, being formed by the 
rotation of an ellipse about the shorter axis. 
The oblateness of the earth refers to the 
diminution of the polar axis in respect of 
the equatorial. The ratio of these two 
axis has been determined in various ways ; 
sometimes by the measures of different de- 
grees of latitude, and sometimes by the 
length of pendulums, vibrating seconds in 
different latitudes. See Earth, Degree, 
&c. 
OBLIGATION, in law, a bond con- 
taining a penalty, with a condition annexed, 
cither'for payment of money, performance 
of covenants, or the like. This security is 
called a specialty. See Bond and Deed. 
OBLIGOR, in law, he who enters into 
an obligation ; as obligee is the person to 
whom it is entered into. 
OBLIQUE, in geometry, something 
aslant, or that deviates from the perpendi- 
cular. Thus an oblique angle is either an 
acute, or obtuse one, i.e. any angle except 
a right one. See Angle. 
OBLiacE cases, in grammar, are all the 
cases except the nominative. 
Oblique line, that which, falling on aa- 
OBS 
other line, makes oblique angles with it, viz, 
one acute, and the other obtuse. 
Oblique planes, in dialling, are those 
which recline from the zenith, or incline to- 
wards tlie horizon. 
The obliquity, or quantity of this inclina- 
tion, or reclination, may be found by means 
of a quadrant. 
Oblique sailing, in navigation, is when a 
ship sails upon some rhumb between the 
four cardinal points, making an oblique an- 
gle with the meridian ; in which case she 
continually changes both latitude and lon- 
gitude, Oblique sailing is of three kinds, 
viz. plain sailing, Mercator’s sailing, and 
great circle sailing. See Navigation. 
Oblique sphere, is where the pole is ele- 
vated any number of degrees less than 90“ ; 
in which case the axis of the world, the 
equator, and parallels of declination, will 
cut the horizon obliquely. 
OBLIQUITY o/ffte ecliptic. See Eclip- 
tic. 
OBLIQUUS, in anatomy, oblique, a name 
given to several muscles, particularly in the 
head, eyes, and abdomen. See Anatomy. 
OBOLARIA, in botany, a genus of the 
Didynamia Angiospermia class and order. 
Natural order of Personatae. Pediculares, 
Jussieu. Essential character: calyx two- 
leaved ; corolla four-cleft, bell-shaped ; sta- 
mina from the slits of the corolla ; capsule 
one-celled, two-valved , many-seeded. There 
is but one species, riz. O. virginica. 
OBSERVATION, in astronomy and na- 
vigation, is the observing with an instru- 
ment some celestial phenomenon, as the 
altitude of the sun, moon, and stars, or their 
distances from each other. But by this 
term, mariners commonly mean only the 
taking the meridian altitudes, in order to 
find the latitude ; and the finding the lati- 
tude from such observed latitude, they call 
“ working an observation.” 
OBSERVATORY, a place destined for 
observing the heavenly bodies : it is a build- 
ing usually in form of a tower, erected on 
an eminence, and covered with a terrace 
for making astronomical observations. Most 
nations have had observatories, which have 
been noticed at large in I^a Lande’s Astro- 
nomy : of these, the following may be men- 
tioned : 
The Greenwich Observatory, or Royal 
Observatory of England. This was built 
and endowed in the year 1676, by order of 
King Charles the Second, at the instance of 
Sir Jonas Moore, and Sir ^Christopher 
Wren : the former of these gentlemen being 
