m 



ASTRONOMY. 



[TUB AZIMUTH INSTRUMENT. 



cope O U again directed to the first object. By fixinc 

 K in tins position by means of the clamp c/, and 

 turning the upper ]>.irt of the instrument until the tele- 

 scope U U <linvt.il to the second object, it is clear that 

 the index of the inner circle has described another arc 

 equal to that already described in the tint operation ; 

 and 10 on, by continuing the aame series of operations 

 until the multiple in sufficiently enlarged, a result may 

 be secured free from error when it is reduced to its real 

 value, whieh is ilono l>y dividing the gross result by the 

 number of multiples, The readings on this instrument are 

 accomplished by secured verniers, whose divisions are 

 enlightened by small plates of polished glass, m m, and 

 by ttie microscopes, n n, trained on the scale ; by those 

 means indications are easily read off. 



THE ALTITUDE AND AZIMUTH IJTSTKCMEXT. This 

 U the most useful of all the portable instruments, 

 and to the scientific traveller an invaluable one, mea- 

 suring with great accuracy both vertical and horizontal 

 angles. (See folio plate). 



In applying the instrument to astronomical purposes, 

 it was formerly the custom to clamp it in the direction of 

 the meridian, and, after taking an observation, or series 

 of observations, with the face of the instrument one way, 

 to wait till the next night, or till opportunity permitted, 

 and then take a corresponding series of observations of 

 the same object with the face reversed, by way of verifi- 

 cation. This is now seldom practised, being obviously 

 imperfect. The instrument consists of a central tripod, 

 to which is fixed the azimuth circle, having a horizontal 

 motion of about three degrees, so that its zero can be 

 brought exactly in the meridian by means of a slow 

 moving screw beneath the circle. The tripod rests upon 

 foot-screws, which are described by Mr. Troughton, in 

 the Memoir* of the Attronomical Society, as "being 

 double ; that U, a screw within a screw, the exterior 

 one having its female in the end of the tripod, and the 

 female of the interior screw being within that of the ex- 

 terior," by which ingenious contrivance, three distinct 

 motions are gained for regulating the azimuth. Brass 

 ups are placed under the spherical ends of the foot- 

 screws ; this screw, invented by Mr. Troughton, is in- 

 tended to give a very slow motion to one of the feet, and 

 the foot of the tripod is designed to be placed either 

 north or south. Above the azimuth circle, and concentric 

 with it, is a strong circular plate, which carries the whole 

 of the upper works. This plate rests on the axis of the 

 azimuth, and moves concentrically with it. Rising from 

 this plate are two strong conical pillars, on which the 

 transit instrument is supported. Upon the axis, as a 

 centre, is fixed a double vertical circle with the telescope 

 between them, the circles being fastened together by 

 mall brass pillars, while the graduated scale is made on 

 a narrow silver ring, inlaid on the outer face of one of 

 the circles. Two reading microscopes are placed at each 

 extremity of the circle, supported by two attached near 

 the top of the pillars. The adjustments required are as 

 follows : The horizontal circle is first to be leveled, 

 which is to be effected in the same manner as with 

 a theodolite. The axis of the telescope must also be 

 leveled, as in the transit instrument, ami the spider-lines 

 adjusted for collimation and vcrtii-illity. The meridi- 

 onal point on the azimuth is its reading when the tele- 

 Mope is pointed north or south. This may be determined 

 by observing a star at equal altitudes, east and west of 

 the meridian, and finding the point midway between the 

 two observed azimuths ; or the instrument may be 

 adjusted to the meridian, in the same manner as a 

 transit instrument The horizontal point of the altitude 

 circle is it* reading, when the axis of the telescope is 

 horizontal, and may be found, as with the mural circle, 

 by alternate observations of a star directly, and reflected 

 from the surface of mercury. The telescope usually 

 carries, in iU principal focus, a spider-like micro- 

 meter, as in the transit instrument, to be described pre- 

 sently. 



THK MicH'iMBTKR. We have had occasion to mention 

 this instrument, and a short account of it may not ba 

 out of place here. It consists of a system of very fine 



wires (Fig. 1!X>), by whieh the apparent magnitudo of 

 Fig. las, heavenly Ixxlics may be measured, 



and the exact moment of their transit 

 across the wires observed with great 

 exactness. In the case of 

 telescopes, the micrometer is 

 in the instrument itself. In circular 

 instruments, and the larger instru- 

 ments of observatories, it forms a 

 part of the microscope attached to 

 the outer edge limb for reading off 

 the graduated scale. 

 When a telescope is fitted with the micrometer, the 



right line of the optical axis o (Fig. 197), exactly coin- 

 Fig. 197. 







cides with the centre 

 receives the ray A, it 

 arrangement of wires, 

 tion may be attained, 

 of the finest platina 

 four transverse wires, 

 with this instrument, 



of the crossed linos B. When it 

 is very obvious that, by a proper 

 the utmost exactness of observa- 

 Tlin micrometer is usually formed 

 wire, having three horizontal and 

 When the telescope is furnished 

 the reticulated frame A (Fig 198) 



Fig. 198. 



is placed at the end of the tube B C, near the eye-piece, 

 which again shuts into the principal tube D of the tele- 

 scope. On the other hand, the object-glass E, F, formed 

 of two lenses, also shuts into the same tube, each re- 

 quiring to have their focal distance from the object-glass 

 adjusted either by the hand or by means of a guiding 

 screw. 



In the case of circular instruments, the micrometer is 

 Fig. 199. fitted in a mici. 



as represented i 

 l'.l!l. It is tirmly fixed 

 in its position, so as to 

 enable the observer to 

 read oif the angle from 

 the graduated scale on the outer side of the limb C D, 



Fig. 200. 



Fig. 200. In small instru- 

 ments, the screws which 

 servo to fix the microscope 

 in its position are usually 

 arranged so as to regulate ita 

 distance from the limb of the 

 circle. In the body of the 

 microscope at A are placed 

 two wires, crossing each other 

 diagonally, which may ba 

 made to traverse the field of 

 view, either horizontally or 

 perpendicularly, by turning 

 the micrometer screw a, work- 

 ing in, the box underneath it. 

 Figure 201 represents tin- field 

 of view, with the magnified 

 bars of the graduated scale aa 

 seen through the microscope. 

 A small mirror, I (Fig. IDS)), is sometimes fixed to tho 

 micrometer, arranged in such a manner as to throw the 



i .-. jut. 



light of a lamp, or a jet of gas upon 

 the part of the limb which is oppo- 

 site to the microscope ; it is some- 

 times pierced with an opening in 

 the centre, which receives the rays, 

 and serves to illumine the micro- 

 scope itself. In the larger in- 

 struments of observatories, they 

 are fixed in the wall which sup- 

 ports the circle, the graduated 

 scale being illuminated by a lamp 



