TRANSIT, OR TKANS1T INSTRUMENT. 



TRANSIT, OR TRANSIT INSTRUJ1 1 



Sll 



pierced on Uw right side to allow the light to pan, and soldered at t 

 and / to the central sphere. The reflecting plate u fixed in toil tele- 

 cope, and can be turned to throw the light up or down. The object- 

 end and eye-end are screwed on at c and d, and are interchangeable. 

 The telescopes are unually in two piece*, which are acrewed into the 

 eentnl iphere at e and /. The advantage expected from the pnwnt 

 construction u, that there is firmer (crew-hold and leas leverage for 

 any blow or rough handling ; and that by interchanging the object and 

 eye-end, fresh portions of the pivoU are brought into action, thus dimi- 

 nishing wear, and equalising minute errors of form or flexure. The 

 object-glass of the telescope should be carefully selected, and of as large 

 an aperture as will show a good image. The superiority of a large 

 instrument over a small one is wholly in the increased optical power. 

 In all other respects it is probably inferior, that is, if the support of the 

 smaller instrument be as solid as that of the larger. There are seven 

 fixed vertical wires at equal spaces, and two horizontal wires, between 

 which the star is observed. The head of the micrometer is shown at //. 

 A small prism for observing stars near the zenith is slipped on the eye- 

 piece when required, as at A. 



The level riiia on the pivots with its T'S. There is a pin at each* 

 end, which drops into a fork at i, to hold the level safely and upright. 

 This ia completely seen at the left pier. At this end u the adjustment 

 for setting the level tube parallel with the axis. At the other end is 

 an adjustment for raising or depressing that extremity of the level. 

 The level should be very sensible and of the same curvature through- 

 out. The graduation we have found moot convenient in to have the 

 principal divisions to 15* and the subdivision to l"-6, numbered as 

 units and tenths, which, though erroneously, is briefly described by 

 calling the units seconds of time. If this scale should be too fine for 

 the level, a principal division to 30", and subdivisions to 8", but still 

 numbered as units and tenths, will be found equally convenient. The 

 riding level is generally applied to the instruments which are so large, 

 and consequently the piers BO high, that a man cannot apply the level 

 safely while standing on the floor, and also to small instruments, of 

 necessity, when they are clamped, as this is, to the pier. For a transit 

 between stone piers which does not exceed five or six feet, we prefer a 

 swinging level, which may be applied and read while standing on the 

 floor. 



When Troughton undertook, much against his will, to construct a 

 ten-foot transit for the Royal Observatory, he adopted a very ingenious 

 mode of uniting the cones and the telescope with the central sphere. 

 The description will be found in the ' Phil. Trans.' for 1826, p. 423 : 

 that part which treats of the construction of the instrument is from 

 Ttoughton's own pen. He also added four braces, to connect the 

 telescope with the axis. We are not disposed to attach much value to 

 this mode of connecting the axis and telescope, which, moreover, 

 requires very accurate fittings. The braces are positively injurious, 

 unless they are exactly and at the same moment exposed to the same 

 temperature. It is said, indeed, in the memoir just mentioned, that 

 when the antagonist braces were exposed to very different tempera- 

 tures, the instrument continued to preserve its form. If so, the expe- 

 riments simply show the centre-work to have been so strong that the 

 braces could not disturb it, in which case they are merely useless. 

 At Cambridge the braces were found to derange the instrument, 

 and were consequently removed, to the great improvement of its 

 steadiness. 



There is no great difference of construction between different tran- 

 sits, except what we have already mentioned. It is desirable even fur 

 the smallest instruments that the supports should be of stone when 

 they are not perpetually shifted about The T'S then are separate 

 pieces fixed by screws to plugs let into the stone. For small transits 

 the stone may be in one or three pieces, according to the size. Wln-n 

 practicable, the piers should bo high enough and wide enough apart to 

 let the observer stand or lie down between them. This saves perpetual 

 meddling with the eye-piece, and the eye is less strained. We have 

 already remarked that the performance of a well-made transit depends 

 rather on the permanence of its fixing than anything else. It is to the 

 greater care bestowed on the foundations of large instruments that 

 much of their superior performance is to be attributed. 



The principal use of a transit instrument is that of determining the 

 exact moment when a celestial body passes the meridian of the place 

 of observation. Now the meridian is a great circle which passes 

 thmtlgh the smith and the pott, and the instrument is adjuxltJ when 

 the lino of sight is a portion of the meridian during the whole rota- 

 tion of the toleacop*. 



As in all other instruments, the telescope is first to be adjusted for 

 distinct vision. Put on a tolerably high power, and slide the eye-piece 

 out and in till you see the wires sharply without straining 

 Then direct the telescope to a bright star or a double star ; and if the 

 image of it is distinct, the telescope la in focus. If not, release the 

 screws at k, and draw the tube out or push it in until the image is as 

 perfect as you can make it. There is another opposite screw to k, and 

 the exterior holes allow a little play. Some trouble and guesswork 

 may be saved by making two slight scratches on the eye-piece where 



W hall pnk >t pmtnt a* U there were only one fixed wire In the 

 tttacopt, aamclr, the middle vertical wire. The iiibttqucnt modification will 



the sight of the wires and of the star are respectively most perfect, and 

 drawing the principal tube out or puahing it in thin quantity. The 

 operation has succeeded, if, in viewing a slow-moving star, like Polaris, 

 there is no shifting between the star and the wire which bisect* it, 

 while the observer moves his head laterally. This adjustment is gene- 

 rally best made by the instrument-maker, and as it is not liable to 

 alter, we should prefer to have the telescope tube cut the proper length 

 upon his responsibility, so that the position of the wire U permanent. 

 When this adjustment is completed, the telescope must be turm 1 on 

 some tolerably distinct object, which is to be bisected by the middle 

 wire near the upper part of the field. If, on raining the telescope, it is 

 also bisected at the lower part of the field, the wire is perpendicular 

 to the axis ; but if not, the tube is to be twisted without altering the 

 focal length until the object comes half-way to bisection. The bisection 

 is completed by the azimuth-screw, when the object ought to be seen 

 bisected at the top of the field when the telescope is depressed. One 

 or two trials will suffice for this purpose, and then the screws at k must 

 be tightened. 



The first of the principal adjustments is that of setting the lino of 

 sight at right angles to the cross-axis, when it necessarily describes a 

 great circle. A distinct object must be selected not far from the hori- 

 zon, and bisected by the middle wire, using the azimuth-screw. The 

 axis is then carefully lifted out of its T'S, and returned end for end, or 

 rtrened, and the object viewed again. It ia now to be bisected as 

 before, half by moving the azimuth-screw, and half by the screw at I 

 and its antagonist, each of which drain the plate on which the wires 

 are fixed. The operation must be repeated until no difference can be 

 seen in the bisection, whichever be the position of the axis. When 

 there is a micrometer, the operation U somewhat easier. The micro- 

 meter wire U brought first on one side and then on the other of the 

 centre wire, so as just to shut out the light between them, and the 

 mean of these two readings is the reading of the zero point, that \\ lii.-li 

 corresponds to the exact superposition of the wires. The distinct 

 object is bisected by the micrometer wire ; and we will suppose that 

 the screw has to be turned three revolutions for this purpose from ita 

 zero position. Now reverse the axis, and suppose the micrometer has 

 only to be moved two revolutions from its zero to bisection. The 

 half-way, or two revolutions and a half, is the distance which should 

 be shown. Carry the micrometer wire two revolutions and a half back 

 from its last position, and that is the position which the centre wire 

 should occupy. If the joint thicknesses of the two wires (previously 

 determined) equal ten parts, carry the micrometer five parts still far- 

 ther back, and bring up the middle wire to touch it by the screws at I 

 as before. The error of collimation is now corrected, as will be found 

 by repeating the observation. The collimating screws have generally 

 a capstan-head, which i awkward, and indeed dangerous so near the 

 eye ; and we think the square head, which is turned by a key, ia more 

 manageable. The screws should be drawn tight, but not forced. We 

 have here followed the usual rule of directing the observer to select a 

 well-defined object near the horizon. If the pivots of the axis are 

 equal, the mark may be considerably elevated or depressed without 

 introducing any error ; and an object which is not very distant niay be 

 seen sharply defined when the aperture of the object-glass is diminished. 

 In fixed observatories there is usually either u meridian mark north 

 and south at a considerable distance, or two near marks, which are 

 made distinct by interposing lenses. At Greenwich and Cambridge a 

 south meridian mark is combined with a supplementary transit in 

 the north slit ; at Oxford two near marks are made visible by lenses 

 fixed in the observatory elite. By using north and south marks at the 

 same time, you are warned of any alteration in the T'S during the 

 reversal, which, when the instrument is heavy, is always to be feared. 

 Such shifting would cause the error of collimation to appear different 

 according as it is taken from one or the other mark. In standard 

 observatories the error of collimation is not actually corrected unless 

 it is large, but measured by the micrometer, as we have described, and 

 the affect thus allowed for by calculation. 



When the illuminated end is west, and the telescope pointing south, 

 let the middle wire appear to be 3'263 Revolutions to the right-hand of 

 the collimating mark : as the telescope reverses, this means that the 

 transit points 3*263 R. to the left of the mark, or to the tail. Now 

 reverse the telescope, the illuminated end is east, and suppose the 

 middle wire still to appear to the right of the mark, but only 3'187 R. 

 These observations are best made on a calm cloudy day ; often, 

 rain, the mark, if dit-tant, is seen sharply and steadily. If there were 

 no error of collimation, the micrometer should give the same quantity 

 at both observations, or & (3'2ti3 H. + 3-187 R), that in, ::-L'L':. K. 

 The error in collimation, therefore, is 0'038 K., and th instrument 

 points to the east that quantity when the illuminated end is west. 

 The scale of the micrometer screw is known. Let one revolution cor- 

 rcxpnud to the space an equatorial star would move over in 3* of time ; 

 then (V038 R. is the space which an equatorial star would move over in 

 0"1 14, which is set down as the error to be employed in calculation. 

 Now the effect of an error of collimation is to make the instrument 

 describe a parallel to a great circle, and distant from it by the collima- 

 tion error. If the instrument -mints to the east of the south, it also 

 points the same quantity to . the east of the north, and a star above 

 pole will pass the a/>/m*( before the true meridian. The correction, 

 therefore, to be applied to the observed transit is +, and when 



