TRANSIT CIRCLE 843 3tly east and west of each other, and rising a convenient height, support the instrument that the telescope revolves freely between lem in the plane of the meridian. This gives le simple transit instrument, by which and necessary accompaniment, the clock, is ob- rved the time of meridian passage (the tran- sit) of the star whose place is to be determined. If now we attach firmly to the axis a finely graduated circle which will revolve with the telescope, we shall be enabled, by means of its divisions, to measure also the precise altitude of the star at the same instant of culmination ; and thus the transit circle will give, by the first observation, the desired right ascension, and by the second, the desired declination of the object. This combination is entirely of mod- ern date. Transit instruments and meridian arcs and circles have been used ever since the days of Roemer and Picard, but the first real conjunction of the two dates from the close of the first quarter of the present century. The sockets of the transit circle receive the pivots and determine the position of the in- strument. They are not formed, as might be supposed, of circular " boxes " accurately fit- ting the pivots, but are simply solid little pieces of gun metal, cut away at the upper surface by two planes inclined to each other like the sides of the letter V, from which letter they take their technical and convenient name. In these V's the pivots revolve smoothly and truly, touching the inclined sides at but two points, and consequently without the lateral play which it would be impossible to avoid in circular boxes, however truly ground. Again, the stone piers upon which the instrument rests, even though wrought into perfect symmetry and equality in every respect, and though posited in such a manner as to furnish no apprehension of relative change, will yet continually mani- fest such change, sometimes under the influ- ence of varying temperature from day to night and night to day, but more frequently from causes even more irregular and less known than this. In order therefore to be able to keep the axis of the instrument duly east and west and truly horizontal, the V's are not perma- nently bedded in the stone, but are so held by strong plates of the same material, themselves permanently fastened, as to allow of small changes of position, one in a horizontal and the other in a vertical direction. Passing next to the telescope, we notice that the narrower ends of the tapering tubes are terminated by flat rings of precisely the same dimensions, upon which are fitted caps containing, one the object glass and the other the eye tube with its mechanisms. These caps are exactly of equal weight, and, partially entering the ends of the tube, their centres of gravity fall truly in the line of junction with the telescope. Thus the instrument is not only perfectly counterpoised, but also, the caps being convertible, the object glass and eye tube may be and should be pe- riodically interchanged, in order to eliminate from an average result the effect of a possible flexure of the tube. The object glass presents nothing worthy of especial remark. The con- struction of the eye piece is peculiar. The term "eye piece" is generally, though incor- rectly, applied to the whole mechanism at the eye end of the telescope, which consists of a small tube sliding in the end cap, and carrying not only the eye piece proper, which is of the form known as Ramsden's (see TELESCOPE), but also a conveniently shaped box containing two thin metallic plates. These plates, called diaphragms, are made with central openings, across which are stretched the threads used to mark the star's position in or its progress through the field. One of these diaphragms is used for the observation of transits, and is securely held in place by fine " antago- nist" adjusting screws. Across its opening and precisely through the centre of the field is stretched vertically a most delicate thread of spider's web, which, as the instrument re- volves, represents to the observer's eye the meridian as a visible line across which the heavenly bodies are seen to pass at the mo- ment of culmination. In order to gain more accuracy in this observation (for the instant of transit is required to be known within a small fraction of a second), other threads are also introduced parallel with the central one and symmetrically disposed on either side of it, so that, by noting the time of crossing each and taking the average, a very great degree of ac- curacy is attained. Ordinarily the transit dia- phragm contains either five or seven threads, all at equal intervals ; but for special purposes their number and arrangement are adapted to the circumstances. "With a telegraphic meth- od of registry, as practised with the large tran- sit instrument of the Washington observatory, five different sets or tallies, with five threads in each, are sometimes used. Across the same diaphragm is stretched horizontally another fixed thread, as a guide to the observer in pla- cing the telescope so that the star shall traverse the centre of the field. The second diaphragm, carrying only a single horizontal thread, is mov- able in a vertical direction between truly fit- ting guides, and by means of a finely wrought micrometer screw. As the first plate belongs to the transit portion of the twofold construc- tion, so this one belongs to and cooperates with the circle, and the office of the screw which carries it is to measure the exact distance of the star, as it traverses the field, either from the fixed horizontal thread, or from some other definite starting point, which may be repre- sented upon the scale of the screw without being necessarily visible. Attached to the screw and revolving with it is a small disk or "head," whose edge is divided into 100 equal parts, so as to measure very accurately the fractions of a revolution, while the whole num- ber of turns necessary to carry the thread to any part of the field is registered upon a con- venient scale usually placed within the eye
