164 ASTRONOMY. 
L’L’ are the two levels, whose end views are given immediately to the left 
hand, with their mode of attachment ; d, d, d, d, e, e, e, e, are the spokes; c, 
c and T, T are hollow tubes. Fig. 5*is a front view of the eye-piece of 
the telescope, CB (fig. 3) ; the letters a, b, c, f, g, h, in figs. 5%, 5°, indicate 
the separate parts of the wire micrometer, and d the pin for its adjustment ; 
c ( fig. 5”) is the separate tube of the ocular. Fig. 6 shows the construction 
of one of the four microscopes, K, attached to the microscope carrier ; figs. 
7, 8, 9, and 10, represent particular parts of the micrometer arrangement. 
Fig. 7 is the inner, fig. 8 the outer plate; fig. 9 the spindle of the micro- 
meter screw h; and fig. 10 the external view of the whole micrometer 
arrangement from above. 
The Equatorial. 
89. The Equatorial is an instrument by means of which, not only the 
declination, but also the difference of right ascension of a star and the 
zenith can be ascertained. Some idea of the instrument may be obtained 
by supposing an altitude circle so arranged, that the axis of rotation, pre- 
viously vertical or perpendicular to the plane of the horizon, shall be per- 
pendicular to the equator; in other words, that this axis placed in the 
meridian shall form an angle with the horizon equal to the height of the 
pole at the place of observation. Thus, in the altitude and azimuth instru- 
ment, the axis of rotation moves towards the zenith; in the equatorial, 
towards the pole of the equator. Its axis thus becomes parallel to that of 
the earth, and the azimuth circle of the simple circle becomes an hour 
circle, and the altitude circle a circle of declination. This instrument, when 
very accurately constructed and adjusted, possesses the exceedingly 
important advantage of giving, out of the meridian, the same determinations 
which the meridian circle affords at the moment of culmination alone. 
The equatorial, as at present constructed, rests upon a prismatic stand. 
In the smaller portable instruments, however, the middle of the polar axis, 
I ( fig. 15, which represents the one constructed by Repsold for the Ham- 
burg observatory), rests upon a vertical brass pillar, A, with three feet ; 
LMN is the hour circle, figured more intelligibly in fig. 17, with its micro- 
meter arrangement, N, and the counterpoise, K. GHF ( fig. 15) is the decli- 
nation circle ; C, D, K, the counterpoises for diminishing the friction ; and OP 
the movable telescope. A very minute division of the hour circle is, strictly 
speaking, not necessary, as the exact determination of the right ascension 
is obtained in another manner. Fig. 16 shows the external and internal 
construction of the axis, AH, of the declination circle, GHF. In a well 
constructed equatorial—l1, the axis of rotation must lie in the plane of the 
meridian, and 2, must form an angle with the horizon equal to the altitude 
of the pole at the place; 3, the plane of the declination circle must be 
parallel both to the axis of rotation and to the optical axis of the 
telescope. 
The stands of the greater equatorials consist of a solid pyramidical base 
164 
