OPT 



ORB 



silvering or past the edges of the same, 

 by one portion of the eye, while the dou- 

 ble reflected object is seen in the silver- 

 ing by the other portion of the same eye. 

 When prismatic reflection is employed, 

 the prism must not be triangular as usual- 

 ly constructed, but quadrangular, and the 

 two reflecting 1 surfaces (to produce an 

 angle of 90 degrees between the first 

 incident and second emergent rays) 

 must make an angle of 135 degrees, 

 while the opposite angle must be one of 

 90 degrees, and the other two angles may 

 be eithar respectively equal or unequal 

 at pleasure ; then one of the faces which 

 make right angles with each other is to 

 be turned towards the object or objects 

 to be delineated, and the rays after pass- 

 ing through that surface and reflection 

 from the two next faces, will emerge 

 from the fourth under the proposed an- 

 gle. The mirrors or other reflecting sur- 

 faces are mounted in a proper frame, and 

 supported at a suitable distance from the 

 paper intended to receive the drawing ; 

 and, when necessary, either a double 

 concave or a double convex glass may be 

 fixed in the frame, and properly adjust- 

 ed to produce distinct vision when the 

 apparatus is used by short-sighted or 

 long-sighted persons respectively. These 

 concave or convex glasses may conve- 

 niently be made of twelve inches focal 

 length ; the instrument must then be sup- 

 ported at the distance of twelve inches 

 from the paper ; a distance which is con- 

 venient enough in other respects. 



Dr. Wollaston has himself published a 

 description of this instrument, in Nichol- 

 son's Philosophical Journal, where he 

 likewise institutes a comparison between 

 the Camera Obscura and the Camera Lu- 

 cida. The objections to the Camera Ob- 

 scura are, 1. That it is too large to be car- 

 ried about with convenience. The Ca- 

 mera Lucida is as small and portable as 

 can be wished. 2. In the former, all ob- 

 jects that are not situated near the cen- 

 tre of view are more or less distorted. In 

 this, there is no distortion ; so that every 

 line, even the most remote from the cen- 

 tre of view, is as straight as those through 

 the centre. 3. In that, the field of view 

 does not extend beyond 30, or at most 

 35, with distinctness. But in the Came- 

 ra Lucida as much as 70 or 80 might be 

 included in one view. 



Dr. Wollaston remarks further, that by 

 a proper use of the same instrument 

 every purpose of the pentagraph may 

 also be answered ; as a painting may be 

 reduced in any proportion required, by 



placing it at a distance in clue propor- 

 tion greater than that of the paper 

 from the instrument, In this case a lens 

 becomes requisite for enabling the eye 

 to see at two unequal distances with 

 equal distinctness ; and in order that 

 one lens may suit for all these purposes, 

 there is an advantage in varying the 

 height of the stand according to the pro- 

 portion in which the reduction is to be ef- 

 fected. 



OPTION, in law, every bishop, whe- 

 ther created or translated, is bound imme- 

 diately after confirmation, to make a legal 

 conveyance to the archbishop, of the 

 next avoidance of such dignity or bene- 

 fice belonging to the see, as the said 

 archbishop shall choose, which is there- 

 fore called an option. 



OR, in heraldry, denotes yellow, or 

 gold colour. In the coats of noblemen it 

 is blazoned topaz ; and in those of sove- 

 reign princes, sol. It is represented in, 

 engraving by small points or dots, scat- 

 tered all over the field or bearing. 



ORATORIO, in music, a species of mu- 

 sical drama, originally an imitation of the 

 serious opera, the subject of which is 

 generally taken from scripture, and can 

 be only treated properly by music of the 

 sublimest style. 



ORBIT, in astronomy, the path of a 

 planet or comet, or the curve that it des- 

 cribes in its revolution round its central 

 body : thus the Earth's orbit is the curve 

 which it describes in its annual course, 

 and usually called the ecliptic The or- 

 bits of all the planets are ellipses having 

 the Sun in their common focus ; in which 

 curve they move according to an invaria- 

 ble law. See ASTRONOMY However, the 

 orbit of the Earth is considerably disfigur- 

 ed by the action of the Moon ; as is also 

 the orbit of Saturn by the action of Jupi- 

 ter, when they happen to be in conjunc- 

 tion. Though the orbits of the planets 

 be elliptical, not circular, yet that they 

 are very little so, even in the most eccen- 

 tric orbit, as that of Mercury, will appear, 

 by comparing their eccentricities with 

 their mean distances from the Sun The 

 orbits of the planets are not all in the 

 same plane with the ecliptic, but are va- 

 riously inclined to it, and to each other ; 

 but still the plane of the ecliptic inter- 

 sects the plane of the orbit of every oth- 

 er planet in a right line, which passes 

 through the Sun, called the line of the 

 nodes, and the points of intersection of 

 the orbits themselves are called the nodes. 



ORCHESTRA, in music, that enclosed 

 part of the theatre -between the audience 



El 17 in SIT 7' 



, v 



