The cradle bar is compound with fine screw movement for perfecting collimation, but to make this adjustment perfect, the telescope tube must be centred in a powerful lathe, and the cradle and solid metal rings in which the tube revolves ([see illustration]) has to be turned and fitted with true flanges or working bearings; the mechanical and optical centres will then coincide.
The hour circle, to be truly divided, must be truly made, and is a solid wheel of good substance, working on a secondary axis, and rotated by mechanical means, truly concentric, with a very strong polar axis.
The declination circle has fine tangent screw movement, with double action for setting the readings by milled-headed screw, and also long driving rod from the eye-piece to set and adjust the object in centre of field. There is slow hand motion, by a separate wheel in right ascension.
The clock-power is connected and disconnected instantly, by touching with the finger a small lever, conveniently placed at a small door in the clock case. The clock will go at exactly the same rate, whether it is driving the telescope or not, and the latter begins to move at the proper speed the instant the lever is moved, and the connection of the telescope and clock made, which is instantly effected by another lever.
| £ | s. | d. | ||
14, 15, to 16 inch Speculum mounted as (Fig. 1), with rotatinghour circle reading to 5 seconds and decimation to 1 minute(the diameter of the circles never less than that of theSpeculum, and often larger) | ||||
8 powers, from 50 to 700, with first-rate and very powerful drivingclock, first-class instrument, and complete | from £250 to | 350 | 0 | 0 |
17, 18, to 20 inch Speculum, with 10 powers, from 50 to 800, includingKellners, Huyghenians, and Achromatics, | from £360 to | 500 | 0 | 0 |
22 to 24 inch Speculum | from £600 to | 700 | 0 | 0 |
30 inch Speculum, with 12 powers and position micrometer, andtransit eye-pieces | 1000 | 0 | 0 | |
Prices will be forwarded for special arrangements in any of these large sizes.
| £ | s. | d. | ||||||
| Driving | Clocks for | 6 | to | 8 | inch | 35 | 0 | 0 |
| 〃 | 〃 | 12 | 〃 | 18 | 〃 | 40 | 0 | 0 |
| 〃 | 〃 | 20 | 〃 | 30 | 〃 | 50 | 0 | 0 |
These are fitted with governor, regulator, and self-adjusting break. They are made of gun metal and steel, and every wheel is cut. They drive with most excellent regularity, and are, when desired, made to beat seconds on a bell. The whole is enclosed in a mahogany frame, with glass panels.
Fig. 2 admits of equal completeness, &c., but the stand is not so convenient for clock-power.
Silvered Glass Equatorial Telescopes, very substantially and well fitted as (Fig. 2). All these equatorials have revolving body.