Class X.j

DOLLONIFS, SIMMS’, AND GERARD’S TRANSIT INSTRUMENTS.

247

telescopes are 14 inches focal length, and 1 • 3 inch aperture,■with altitude circle and the regular transit series of spiderlines, supported upon four pillars fixed to the horizontalcircle.

When the instrument is used for determining the actualor diurnal variation, there is a cover containing a lens,which is applied in front of the object glass of the tele-scope for reading off the divisions on the needles, of whichthere are two, 7 inches in length, together with variouseye-pieces, &e. The instrument is supported on a strongtripod with adjusting screws.

Simms (741, pp. 475*—477*). A transit instrument,intended for use either in the meridian or prime vertical.The telescope and axis are of the form usually employedfor this class of instrument, viz., two stout cones are con-nected by a sphere into which the cylindrical tubes of thetelescope' are screwed; the pivots are of hard bell-metal,and are both perforated.

The object-glass has an aperture of 2J inches, and afocal length of 42 inches; the diaphragm is furnishedwith seven vertical and two horizontal spider-lines. Thereis another vertical spider-line carried by a micrometerscrew, and which can he moved completely across thefield, so that the distance of an object from any one ofthe fixed lines may be measured by its means. The linesare adjusted to the focus of the object-glass by a milledhead near the eye-end, and another milled head is placednear the axis for the purpose of regulating the intensityof the illumination of the field of view.

The axis of the instrument is converted into a telescope,by having an object-glass fitted into one of the pivots,and a sliding tube carrying an adjustable cross of delicatespider-lines with a positive eye-piece in the other.

The adjustment of this axis telescope is effected by asmall collimator placed in a line with the axis—(unless adistinct mark can be seen through it),—and then turningthe axis round, and moving the proper screws until theintersection of the lines remain stationary upon the mark,or collimator cross, during an entire revolution. If theprincipal telescope be adjusted to the meridian, the markseen through the axis will be 90° distant in azimuth, andbv its means the instrument can immediately be placed inthe prime vertical, for the determination of the latitudeby observation of stars near the zenith. By this inventionthe form of the pivots may be examined, for if they benot circular, the error will be shown when the cross isbeing adjusted. The azimuthal adjustment is providedwith a micrometer head, showing seconds of arc, so thatwhen the delation is of known amount, the instrumentmay at once be placed exactly in the meridian. To takea complete observation of a star in the prime vertical, itis necessary to reverse the instrument, quickly and deli-cately, for a slight concussion might disturb the adjust-ments, and consequently vitiate the observation. Forthis purpose a reversing frame has been contrived andfitted to the base of the cast-iron stand, or support uponwhich the instrument is placed. It consists of a strongscrew having square threads, the circumference of whichfits into a cylindrical hole bored through the centre ofthe stand, and is prevented from turning round by a keywhich fits into a groove cut longitudinally throughout thelength of the screw. The screw is raised or depressed bya pair of bevelled wheels, one of which fits upon the screw,and the other is worked by a winch handle. Above thescrew a stage is fixed, and upon this two conical pillarsare erected, having a fork covered with vulcanized Indiarubber upon the top of each of them. Upon turning thewinch, the screw is raised, and the forks are brought intocontact with the transit axis, and by continuing themotion, the instrument is lifted out of its Y bearings.The frame with the instrument is then, by means of stops,which are attached to the stand, to be turned 180° inazimuth, and then by reversing the motion of the winch,lowered again into its bearings. Two setting circles of4 inches diameter, and divided to one minute of arc, areattached to the eye-end of the telescope, both of which,previously to an observation, should be set to the zenithdistance of the star, in order that no time be lost afterreversing the axis, in again directing the telescope to theobject.

The stage is forced upwards by means of four spiralsprings, which rest upon the base of the stand; these aresufficiently strong to support the whole weight of theinstrument when lifted, so that the friction upon themoving parts is very inconsiderable.

The eye-pieces with this instrument are six in number,namely, five direct, magnifying 40, 53, 86, 120, and 140times, and for zenith observations a diagonal eye-piece,magnifying 90 times.

Mr. Simms also exhibits a diagonal transit instrument,of the form used by Reichenbach in his altitude andazimuth instruments. The object-glass is 1*7 inch indiameter, and has a focal length of 18 inches, with magni-fying powers of 25 and 45 times. The circle, which isintended for a finder, is 6 inches in diameter; is graduatedto show by opposite verniers to one minute of arc; a spiritlevel is attached to the index of the circle, and there isalso a sensitive spirit level for the rectification of the axis.The whole is mounted upon a light cast-iron stand, and issuitable for fixing at a chamber window. In this con-struction, as is well known, the rays which pass throughthe object-glass, do not proceed directly to a focus, butare reflected by a prism, or speculum, within the axis, andform an image within one of the pivots, which is per-forated, and in which the diaphragm and eye-piece areplaced.

Up to the time of Mr. Simms’ adopting this mode, nomethod for the illumination of the field of such an instru-ment had been devised, except the old one of placing areflector in front of the object-glass, and throwing lightupon it by means of a lamp or candle, in a distant part ofthe room in which the instrument was fixed. There areseveral objections to this method;—first, the difficulty ofthrowing light at all upon the reflector under such cir-cumstances ; secondly, the trouble of having to re-adjustit when the direction of the telescope is changed; andthirdly, part of the object-glass itself is cut off by thereflector being placed in front of it. Mr. Simms has veryingeniously overcome these difficulties by lighting up thefield of such an instrument, through the opposite pivot tothat in which the diaphragm is placed, which obviates allthese objections, and effects the object as easily, in thiscase, as in that of an ordinary transit instrument. Thedetailed means of doing this are as follows:—

The back pivot, being perforated, has a small convexlens set within it, and a large convex lens of suitable focallength is fixed within the axis, at the back of the reflector;and its diameter is such that one or more segments nearthe edges (very small segments are sufficient), projectbeyond the edge of the reflector, and a lamp is placed atthe back, with its light directed through the pivot. Theaction of the lens within the pivot condenses the light,which, after crossing, diverges upon the large lens at theback of the reflector, the open segments of which againcondense it upon the field of view, where it produces afull and abundant illumination, that remains unchangedby any variation in the direction of the telescope.

Gerard (109, p. 418) has exhibited a portable or fieldtransit instrument.

The instrument consists of two mirrors, so fixed as tomeasure an angle of 90 degrees. In use two stations areselected, the one about 100 yards east or west of the other,and on the same level. A mark resembling a target withequidistant concentric circles, is set up at one of thestations, the instrument taken to the other, and so placedas to have the mark in the middle of the field. Thetelescope is then turned on its own axis till the sun orstar is seen by reflection, which will be a few minutesbefore it passes the meridian. The times of its passingthe different circles in succession should be noted. Anotherset of similar observations should be made at the secondstation, the mark being set up at the first. The time ofthe meridian passage will be the mean of all the times atboth stations.

Dollond (145, pp. 426—42S, and see Illustration) hasexhibited an altitude instrument, with two circles of 2 feetdiameter divided on silver to every five minutes, eachcircle reading oft' to seconds by means of two micrometermicroscopes attached to the cones which support theinstrument. Two small circles are fixed towards the eye-