TRA NSIT, or TRANS IT I NST RUM ENT (Instrument des Passages), was Invented by !Rimer about the year 1690. The description is to be found at page 47 of the Basis Aatronotnine/ by his Horrebow, Havnite, 1735; and we recommend the perusal of this book, which contains an account of it6mer's inventions and methods, to all those who, reading Latin with moderate ease, fuel a desire to learn the origin of modern practical astronomy, The object of the present article In to give such an account of the transit as will enable any one to use it with tolerable success. Those who wish for more perfect information must consult the introductions to the Oreenwich, Konigsberg, Dorpat, Cambridge, Edinburgh, &e., Observations,' Our type will be the portable transitinstrument, leaving the reader to accommodate what is hero said to the powers of his own instrument, or to the practice of the Observatory which he adopts for a model.
There are three principal parts expressed in the cut. The iron stand, carrying the 'e with their adjustments ; the telescope, inserted at right angles through an axis with a small vertical circle for finding or verifying stars ; and the cross level The stand is made of cast iron, and should be of great strength, though perhaps that which is hero figured would be found inceinveniently heavy if the instrument is often moved. The Y's are contained in brass pieces, strongly united to the tope of the two uprights. The left hand Y has a motion up and down, which is given by a milled screw partially seen immediately under the pivot. The right hand Y is moved in azimuth by a screw, the milled head of which is seen projected upon the lantern. In port able instruments it is very convenient to have this lateral or azimuthal adjustment made by screwing against a spring, as it is in this instru ment. In fixed observatories the adjustment is made by two antago nist drawing screws, one of which is tightened and the other loosened; and indeed this is the general construction of imstrumente of every size, and is the most solid fixture. But it is so convenient to be able to move the instrument at pleasure in azimuth while actually looking through the telescope, that we should strongly recommend the adoption of the counter-spring whenever the instrument is small, and is either to be frequently shifted, or is not furnished with a meridian litark. The spring must press pretty strongly against the screw, and there should be a clamping button in each adjustment, to keep all secure.
The axis is made of two strong brass cones soldered on the central sphere. The sphere in cast hollow with two shoulders, over which the cones slip. As this is the most important part of the instrument, great care should be taken of the fitting before the axis is finally put together, and the symmetry of the parts as to the centre should be per feet. If the instrument is weak here, it is utterly worthless. In the older English instruments' the centre was a cube, and that form is fre quently adopted at present by contineutal artists. The transit at Bruxelles, by Gambey, one of the largest and finest instruments iu the world, is so constructed. The essential requisite, however, is sym metrical strength, and any shape is good which fulfils this condition. The pivots are soldered into the extremities of the cones, and are turned after the whole is fixed. One of them is pierced to admit light into the axis. In large instruments the pivots have an cuter surface of steel, which is less affected by wear. Greater care is required to guard steel pivots from rust,' and the turning must be performed with a diamond cutter, as tho hard knots to which steel is subject resist and jar the ordinary cutter out of its place. The pivots should be turned pretty nearly to the same diameter : the marks of the tool are ground off afterwards by collars which are made to fit closely on the pivots, and are changed and reversed from time to time. When the surface is perfectly formed, the grinding should be discontinued, as a small difference of size in the pivots is of little consequence, while an altera tion of the cylindrical form of the pivots, or of the direction of their axes, ruins the instrument,. The perforated or illuminated end of the axis is on the right hand pier in this figure. The light of the lantern shines through this, and lights up an annular plate in the centre, which makes an angle of 45' with the axis and with the telescope, and thus light enough is thrown down to the eye-end to illuminate the field very vividly, while the opening allows the rays from the object-end to pass without impediment. The quantity of light may be regulated by a
contrivance for diminishing the aperture of the lantern, or by a shade passing between the lantern and the pivot. In some transits there is a contrivance for altering the angle of the central reflector in the body of the instrument ; but this, although very handy, is objectionable, 118 affecting the symmetry of the instruaneut, In a thirty-ineh transit thu lantern is within reach, and may be twisted a little, so as to reduce the light at pleasure. The setting circle, with its level and clamp, are towards the illuminated end of the axis. The tail-piece, which is attached to the verniers and level, is held between the rounded ends of the two screws at a. By screwing one and loosening the other, the bubble of the level is brought to the middle, when the vernier points out the reading of the circle. There is a lens and reflector, for lighting and reading off the circle. The instrument here figured has a vernier which reads single minutes ; but the vernier is inconveniently long for a fixed lens, and we should prefer reading to every 2', which is more than sufficiently near for finding or identifying stars. If the small circle is carefully looked at, two out of three small screws are seen which fix the circle to the axis. When these are released, the circle will turn freely round. Thus contrivance will save some trouble when an Instrument is used for a long time in the same place without reversing, but is scarcely worth being app/i xi to one which is fre quently shifted or reversed. The clamp for fixing the telescope in altitude and the slow-motion screw are seen at b. There is a caution to be given here. The tail-piece should never be tightly nipped, unless the instrument is used for observing declinations, and it and the tangent screw must be released when the observer uses the azimuth screw for bisecting any object, such as a mark or a star at a given moment. In large transits there are generally two small circles fixed on each side of the transit towards the eye-end. They are here more convenient for setting, and it is easy to pass rapidly from one star to another, when both the circles aro previously set. There is great diversity in the graduation of the setting circles. In large instruments which are used for some time in the same position, it is best to make the verniers rend polar distance or declination, re-adjusting the circles whenever the transit is reversed. With a portable transit, which is or ought to be very frequently reversed, a graduation to altitudes one way, which becomes zenith distances when reversed, is perhaps as convenient as any, though a slight computation for ea ,h star is required to form a working catalogue. The telescope in this instrument is not inserted in the ordinary manner. The central portion, from c to d, is in one tube, pierced on the right aide to allow the light to parr, and soldered at e and / to the central sphere. The reflecting plate is fixed In this tele scope, 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 usually in two pieces, which are screwed into the ceutral sphere at e and J. The advantage expected from the present construction is, that there is firmer screw-hold and less leverage for any blow or rough handling ; and that by interchanging the object and eye-end, fresh portions of the pivots ere brought into action, thus dirni niahing 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 g. A small prism for observing stars near the zenith is slipped on the eye piece when required, as at h.