OBL 



OBS 



ence into six equal parts, pricking the 

 points of division through with a fine 

 needle ; cover one side of the glass with 

 this paper, and, exposing it to the sun, 

 receive the rays through these six holes 

 upon a plane ; then by moving the plane 

 nearer to, or further from the glass, it will 

 be found whether the six rays unite ex- 

 actly together at any distance from the 

 glass ; if they do, it is a proof of the regu- 

 larity and just form of the glass; and the 

 said distance is also the focal distance of 

 the glass. A good way of proving the 

 excellency of an object-glass, is by placing 

 it in a tube, and trying it with small eye- 

 glasses, at several distant objects; for 

 that object-glass is always the best which 

 represents objects the brightest and most 

 distinct, and which bears the greatest 

 aperture, and the most convex and con- 

 cave eye-glasses, without colouring or 

 haziness. A circular object-glass is said 

 to be truly centered when the centre of its 

 circumference falls exactly in the axis of 

 the glass ; and to be ill centered when it 

 falls out of the axis. To prove whether 

 object-glasses be well centered, hold the 

 glass at a due distance from the eye, and 

 observe the two reflected images of a 

 candle, varying the 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 proved whether they be well 

 centered by fixing the tube, and observ- 

 ing, while the cell is unscrewed, whether 

 the cross-hairs keep fixed upon the same 

 lines of an object seen through the teles- 

 cope. 



OBJECTIVE line, in perspective, is any 

 line drawn on the geometrical plane, 

 whose representation is sought for in a 

 draught or picture : and the objective 

 plane is any plane situated in the horizon- 

 tal plane, the representation of which is 

 required. See PERSPECTIVE. 



OBLATE, flattened, or shortened, as 

 an oblate spheroid, having 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 axes has been determin- 

 ed in various ways ; sometimes by the 

 measures of different degrees of latitude, 

 and sometimes by the length of pendu- 

 lums, vibrating seconds in different lati- 

 tudes. See EARTH, DEGHFE, &c. 

 VOL. V. 



OBLIGATION, in law, a bond con- 

 taining a penalty, with a condition an- 

 nexed, either for payment of money, per- 

 formance of covenants, or the like. This 

 security is called a specialty. See BOND 

 and DEEP. 



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 perpen- 

 dicular. Thus an oblique angle is either 

 an acute or obtuse one, i, e. any angle ex- 

 cept a right one. See ANGLE. 



OBLIQ.UE cases, in grammar, are all the 

 cases except the nominative. 



OBLIQTTE line, that which, falling on an- 

 other line, makes oblique angles with it, 

 viz. one acute, and the other obtuse. 



Op LI QUE planes, in dialling, are those 

 which recline from the zenith, or incline 

 towards the horizon. 



The obliquity, or quantity of this incli- 

 nation, or reclination, may be found by 

 means of a quadrant. 



OBLIQ.UE sailing, in navigation, is when 

 a ship sails tipon some rhumb betweenthe 

 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 uphere, is where the pole is 

 elevated 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 of the ecliptic. See Ecu*. 



TIC. 



OBLIQUUS, in anatomy, oblique, a 

 name given to several muscles, particu- 

 larly in the head, eyes, and abdomen. See 

 ANATOMY. 



OBOLARIA, in botany, a genus of the 

 Didynamia Angiospei-mia class and order. 

 Natural order of Personatae. Pediculares, 

 Jussieu. Essential character : calyx two- 

 leaved ; corolla four-cleft, bell-shaped ; 

 stamina, from the slits of the corolla; 

 capsule one-celled, two-valved, many- 

 seeded. There is but one species, vi.z. 

 O. Virginica. 



OBSERVATION, in astronomy and 

 navigation, is the observing with an in- 

 strument some celestial phenomenon, as 

 the altitude of the sun, morn, and stars, 

 or their distances from each other. But 

 by this term, mariners commonly mean 

 only the taking th* meridian altitudes, in 

 order to find the latitude ; and the finding 



