243
TRANSIT CIRCLE, EQUATORIAL INSTRUMENTS, Ac.
[Class X.
end. of the telescope, each of which reads off the minutesby verniers, with levels for setting the instrument in alti-tude. The telescope is of 2f inches aperture and 32 inchesfocal length, and is fixed between the two large circles.There are five vertical and three horizontal spirit levelsin focus of the object-glass, also various eye-pieces, &c.
The instrument is supported in Ys fixed on the top oftwo strong cones, which rest on a circular metal base,with differential screws for levelling. The usual methodof illuminating the webs through the axis of the instru-ment is adopted. There are also two fine spider linesreading to seconds.
Mr. Dollond also exhibits a double altitude instrumentfor observing by reflection and direct vision* (Cat. supra).The altitude circles are 1 foot in diameter, divided to 20',and arc read off by micrometer microscopes showingseconds. Each circle has a telescope of 15 inches focusand 1*3 inch aperture attached to it, and there are twosmall circles, read by verniers, for setting the instrumentin altitude.
The azimuth circle is 1 foot in diameter, and reads offby verniers to every 10", having a telescope fixed under-neath as a check upon the instrument when taking hori-zontal angles. The axes of the circles are perforatedin the usual manner for illuminating the spider lines.The instrument is furnished with fine spirit levels, eye-pieces, &c.
Simms (741, p. 47G*). A transit circle, the telescopeof which has a focal length of 4 feet, and an aperture of3*25 inches, furnished with a micrometer in the eye-piece,and magnifying powers of G3, 102, 130, and 165 times.The circle is 2 feet in diameter; it was cast in one piece,of bell-metal. It was divided by Mr. Simms’ self-actingengine into spaces of 5' of arc, and subdivided into singleseconds by micrometers, of which there are two, fixed tothe stone piers which support the instrument.
In addition to the finer graduations, there is anotherstrong circle of divisions, which are cut upon the bell-metal, and numbered to four times 90, showing citheraltitude or depression. These divisions are read by apointer, and they serve to direct the telescope to the object,whether it be seen by direct vision or by reflection.
The Y supports are fixed upon the stone piers. Oneof the Ys has a vertical adjustment for levelling the axis,which is performed by means of a striding level, and theother has an adjustment in azimuth for adjustment to themeridian. There is also a lever counterpoise fixed to thatend of the axis near to which the circle is fixed for thepurpose of equalizing the pressure of the pivots upon theirrespective supports. The instrument is furnished withan embracing clamp, or one which lays hold of the axis,to distinguish it from that kind which bites at the circum-ference, and also with a tangent screw, which exerts nolifting power, and has, therefore, no tendency to disturbthe meridian position of the telescope.
The arrangement for the illumination of the wires ispeculiar, and is the invention of the Astronomer Royal;but it was greatly facilitated by adoption of the peculiarmethod of graduating the light in the field of view, whichhad been practically introduced by Mr. Simms.
The illuminator, which is, as usual, placed near thecentre of the transit axis, and receives the light throughone of the pivots, turns upon an axis in order to changeits angle of inclination with respect to the eye-piece. Bythis change of inclination the quantity of illumination isregulated; and if the plane of the reflector be placed per-pendicular to the axis of the telescope, no light whateverwill be reflected from it. Upon the reflector one or moreprisms are so placed as to receive the light from the lampwhen the reflector is in darkness. These prisms reflectthe light when the reflector is in the position above men-tioned, but at no other time, to other prisms, which arefixed in the eye-end of the telescope, near to the diaphragm;and by these last prisms the light is thrown obliquelyupon the spider lines, and produces the effect of illumi-nating the wires on a dark field. To the edge of thereflector a wire is attached, which terminates in a smallknob, or handle near to the eye-piece, and therefore, near
* See i; Astronomical Society’s Papers,” Nov. 24th, 1823.
the observer’s hand. The only operation required inpassing from one state of illumination to another, with allthe intermediate gradations, is either to draw out or topush in a knob or handle through a space of one or twoinches.
By these arrangements power is given to the observerto regulate the quantity of light in the field of the tele-scope, from the faintest gleam, by which the wires arejust made visible, to the full illumination, and also tochange instantaneously, as circumstances require, a brightfield and dark lines to a dark field and bright lines. Thischange has hitherto been effected by the employment oftwo lamps, one placed near the perforated pivot to givethe ordinary illuminated field, and the other to applyoccasionally at the eye-end of the telescope, very nearlyin the plane of the diaphragm, so that the light passingthrough a slit obliquely across the field of view, illumi-nates the lines which intercept its passage, leaving thesurrounding field in darkness. It is clear that the changefront one kind of illumination to the other with thesearrangements, requires some considerable time, ami thehanging a lamp on the eye-end of a telescope of such aninstrument as a transit circle is exceedingly objectionable.By the new invention one immoveable lamp either illumi-nates the field or the wires, and the same lamp, at thesame time, also illuminates the graduated circle by meansof prisms attached to the micrometer microscopes.
This instrument has also another improvement, whichwas suggested by Mr. W. Simms, jun., since the erectionof the transit circle at the Royal Observatory, Greenwich .It is by the perforation of a hole, about one inch and ahalf in diameter, through the centre cube of the axis, atright angles to the axis of the telescope, in order that anorth and south collimator may look into each otherwithout lifting the instrument from the Y bearings, andby this means very much simplifying the act of adjustingthe line of collimation.
Ross (254, pp. 435, 436) exhibits a large equatorial.
The focal length of the telescope is 18 feet, and theaperture is ll£ inches. The diameter of the hour-circlcis 2 feet 3 inches, and that of the declination circle is2 feet 8 inches.
The instrument is supported on a round cast-iron pillar10 feet 9 inches in height, 2 feet 3 inches in diameter atthe bottom, gradually decreasing to 1 foot at the top. Itis formed of two portions, fastened together at the heightof 4 feet 6 inches from the bottom of the base, by eightscrew-bolts and nuts, passing through flanges, 3 inches inwidth, from the shaft of the column. This joint affordsthe means of an approximate meridian adjustment. Thebase of the pillar is 9 feet in diameter.
The polar axis is of cast-iron, 6 inches in diameter, and5 feet in length; it is connected to the declination axisby a flange of 18 inches in diameter. The length of theinner male declination axis is 3 feet 6 inches between itsbearings; and the outer or hollow axis is 7 inches indiameter. Both are of cast-iron. The inner axis and itsflange form one casting with the central hollow cylinder, tothe flanges of which the corresponding gun-metal flangesof the telescope are bolted. The tube is of copper.
The fitting bearings of the declination axis arc cylin-drical, and the axis is secured by a steel collar 4 inches inlength, fastened by cross pins to the male centre at theend to which the circle is fixed. In the inner axis thereis a counter-sunk cavity to receive this collar, and a secondcounter-sunk cavity, of large diameter, to receive a steelplate, which is fastened against the end of the steel collarby eight steel screws. By this means the end shake ofthe axis is adjusted.
The declination circle is of gun-metal, and is regulatedby an endless screw, having an eccentric lever attached toa dovetail slide for gearing, and a pair of bevelled wheelsat either end for Hooks’ joint adjusting rods. There are,also, two other radial arms, with clamp screws, for securelyfixing the telescope to the circle.
The upper part of the polar axis fits into a couplingblock, having a hemispheric bottom; this is supported byan angular projection from the top of the pillar having acorresponding nearly hemispherical cavity, and the wholeis bound together by bolts and nuts, having spherical