QUADRANT. 



viewed in making a contaft, otherwife both the image of 

 one objeft, and the fubftancc of the otlier, could not be 

 feen at the fame inllant. F is the fight-vane, witli two 

 holes, ufually an upper and a lower, inferted mto the firll 

 radial bar of the frame, fo that the holes are at the fame 

 dillance from the plane of the inftrumcnt as the line of fepa- 

 ration is in the half-filvered glafs E, to prevent parallax of 

 the refledled rays, and in the bell inllruments this vane, 

 with its telelcope, has an adjuilable motion to and from 

 the plane, in order that more or lefs liglit may fall on either 

 of the bodies obferved, which is not only ufeful, but nec.-i- 

 fary, in taking a lunar diftance ; for by this adjuilment the 

 image of one body may be made as luminous as the real 

 body of the other, by increafing the light of one, while it 

 decreafes that of the other ; that is, by making more of 

 the filvered, or of the unfilvered part of glafs E, fall be- 

 fore the objeft-glafs of the fmall tclefcope, when this ad- 

 juilment is neceifary. It may be neced'ary to obferve here, 

 that every filvered glafs of fenfible thicknefs has two re- 

 fledlions, one on the anterior, and the other, which is the 

 principal one, on the pofterior or filvered face, and in many 

 cafes errors may be occafioned by thcfe double refleftions ; 

 to remedy which. Dr. Mafl<elyne propofed, that the filvered 

 portion of the pollerior face fliould be ground rough 

 and painted black, taking care that the grinding be fo 

 performed, that the line of feparation between the polifhed 

 and unpolifhed parts be parallel to the plane of the oftant. 

 In the back horizon-glafs, which is that feen with its light- 

 vane at G, with adjuftments fimilar to thofe of xW fore 

 horizon-glafs at E, the whole pollerior face is filvered, ex- 

 cept a flit, that divides it in a line parallel to the ))lane of 

 the frame, through which flit the body is oblerved in a 

 back obfervation. But when the fun is the body obferved, 

 his light is generally too intenfe for the eye to bear, par- 

 ticularly when the fmall telefcope is ufed ; to render the 

 rays tolerable to the eye, a coloured glafs, or glafles, mufl 

 be interpofed, and a blank tube mftead of a telefcope be 

 ufed, by which means the fun may be viewed without doing 

 injury to the eye ; and ufually a fyftem of coloured glafles 

 are Aided into a fquare hole, as at H, and a ioint in each 

 allows them, or any one of them, to be brought forwards 

 into the fituation where the direft rays muft pals to the eye. 

 The fame remedy is alfo fometimes applied at the eye-pi-ce 

 of the blank tube, when the fun alone is obferved. The 

 plane of the back horizon-glafs is placed not only parallel 

 to the plane of the frame, but at right angles to the plane 

 of the fore horizon-glafs, in order that the fame readings 

 may apply in both kinds of obfervation, and this ob- 

 jeft was effefted by the maker bv the fimple reftangular 

 pofition of the two horizon glafles to each other, and by 

 reverfing the pofition of the body in making a back 

 obfervation ; for as the inclination of the index-glafs to the 

 fore horizon-glafs gives double the angle direftly meafured ; 

 fo twice the complement of this inclination to the back horizon- 

 glafs gives the fame quantity, and on the fame part of the 

 graduated \\v.\h, when meafured in a reverfed pofition. 



Adjujiments. — The adjuftments of every inilrument are of 

 the utmoft importance to the accuracy of obfervations taken 

 thereby, but they are particularly fo in the inilrument be- 

 fore us, becaufe a fmall deviation from parallehfm in the 

 fore horizon-glafs, or from perpendicularity in the back 

 horizon-glafs, as compared with the index-glafs, fwith the 

 vernier at zero of thehmb,) will double the error occafioned 

 by it ill the quiefcent iUuation, as foon as motion is given 

 to the index-glafs. i. The firll adjuftmeut is, to fet the 

 index-glafs perpendicular to the plane of the inflrumcnt : this 

 k done by firll Aiding the index to about 40° or 45° of the 



10 



limb, while the oftant is held with its plane nearly parallel 

 to the horizon, then a glance into the index-glafs will fliew, 

 wliether the (harp edge of t)ie limb feen by reflcftion, isanexaci 

 and llraight continuation of the fame edge of the.limb feen 

 by direft vifion, and if this is not the cafe, it muft be made 

 fo by the fcrews that adjull the bed of the mirror ; and in 

 all cafes where two fcrews are to be ufed, one mull be re- 

 leafed as much as the other is fcrewed inwards. Dr. 

 Mackay propofes to place two fmall pieces of metal as ad- 

 jufting tools, with each a horizontal line drawn on it, at 

 equal heights from the plane of the limb, at fome dillance 

 from each other, fo that the horizontal line of one may be 

 a continuation of the horizontal line of the otlier, when 

 one is viewed by refleftion, and the other by direft vifion, 

 but the fliarp edge of the inner part of the limb will anfwer 

 all ordmary purpofes. 2. The fecond adjuftment is, to fet 

 the fore hor'tccon-glafs perpendicular to the plane of the infirument : 

 this is done by firll fixing the zero of the vernier at zero on 

 the limb, and while the plane of the frame is held parallel 

 to the horizon, by applying the eye to the lower hole of its 

 fight-vane j then if the horizon, or any dillaut horizontal 

 line, appear to be a ftraight line, the glafs is placed perpen- 

 dicular already; but if the parts leen by refleftion and by 

 direft vifion do not conililute a ftraight line, they mull be 

 made to do fo by the fcrews at the bed of the glafs, one or 

 other of which muft be fcrewed in, as the frafture of the 

 line demands ; that is, if the line feen by refleftion appear 

 above that feen by direfl vifion, that fcrew muft be urged 

 inivards which, is on the further glafs, and vice verfd, until 

 the fraftured line be ftraight. 3. The third adjuilment is, 

 to reBify for the index-error of the fore horizon-glafs, or to 

 place it parallel to the index-glafs, when the zero of the 

 vernier is placed at zero on the limb. To do this properly, 

 a bright diftant objeft muft be chofen, fuch as the fun, moon, 

 or ftar, and while the zeros coincide, look through the 

 vane, or telefcope, and obferve if the body and its image 

 coincide ; that is, if the image is invifible : in this cafe, the 

 fore horizon-glafs is parallel to the index-glafs, or is truly 

 adjufted ; but if not, releafe the tail-piece or lever behind 

 this horizon-glafs, and with the thumb-piece give it a fmall 

 motion, till the body and its image coincide, in which fitua- 

 tion it mull be fixed by the fixing nut before releafed. 

 Sometimes the aft of fcr^wing will difplace in fome degree 

 the tail-piece, and thereby again occalion a fenfible index- 

 error. When this cannot be completely avoided, the error 

 muft be afcertained and allowed for with its fign + or — 

 in ever)' fubfequent obfervation, while it remains unaltered. 

 This error may be afcertained by meafuring the diameter of 

 the fun, firll forwards on the limb, and then backwards on 

 the arc of excefs, and one half of the difference of the mea- 

 fures will be the index-error + or — , as the cafe may be. 

 4. The fourth adjuftment is, to fet the back hori^^on-glafs 

 perpendicular to the plane of the infirument : this adjuftmeut is 

 fimilar to the fecond, and niay be performed by the direc- 

 tions there given, fuppofing them to be for the back horizon- 

 glafs and its vane, inftead of the fore horizon-glafs. 5. The 

 fifth and lait adjuftment is, to fet the back horizon-glafs per- 

 pendicular to the plane of the index-glafs produced, the zero of 

 the vernier being placed at zero on the limb: this adjuilment at 

 fea is performed thus ; let the vernier be put as much to the 

 right of zero, as is equal to twice the dip of the horizon, 

 in the fituation where the obferver ftands, then hold the 

 quadrant in a vertical pofition, and apply the eye to the 

 back-horizon vane : now, if the horizon feen by refleftion 

 happen to coincide with that feen by direft vifion, the glafs 

 IS already i-ight ; but if not, tiie lever or tail-piece beliind 

 the frame at this place mull be releafed by the proper fixing 



fcrew. 



