394 SEC. 11. ASTRONOMY. 



b. ALTAZIMUTH INSTRUMENTS. 



1763. 12-inch Altazimuth. Trougltton fy Simms. 



Circles, 12 inches in diameter, divided into spaces of 5' arc; by means of 

 the attached microscopes these spaces are further subdivided, one division 

 upon the micrometer = 1" arc. 



1763a. Pocket Altazimuth on Small Stand for altitudes, 

 compass bearings, &c., improved by Francis Galton, F.R.S., and 

 specially adapted for travellers. L. Casella. 



1763b. Altazimuth Instrument, made by Dollond wjth 

 double altitude circles. W. Watson and Son. 



The instrument stands on three adjusting foot screws, above which is the 

 azimuth circle with a telescope attached. The azimuth circle is an arrange- 

 ment of one circle turning within another, so made that their upper surfaces 

 are in the same plane, and the inner edge of the larger in contact with the 

 outer edge of the smaller. The outer edge of the smaller circle carries the 

 divisions, and the inner edge of the larger circle three verniers ; the gradua- 

 tion is to 10 minutes of arc; above these is the large conical axis of the 

 azimuth circle, from the top of Avhich spring rectangular arms carrying the 

 altitude circles, and telescope. The telescope is mounted between two 

 12-inch circles, the peripheries of which are divided on silver to 10 minutes 

 of arc, each circle has two verniers and is furnished with tangent screw move- 

 ments. Lamp with graduating aperture. 



1764. Comet Seeker, on a stand, with horizontal and vertical 

 motion, constructed by Professor Kaiser. 



H. G. von Bakhuyzen, Director of the Observatory, 

 Leyden. 



The stand possesses the advantage that the eyepiece, which is fitted with a 

 total reflecting prism, remains at the same height in all the positions of the 

 telescope, whilst the axis of the eyepiece remains horizontal. 



4554. Photograph of the Vertical or Zenith Telescope, 



for the observation of stars culminating near the zenith. 



Prof. Lorenzo Respighi, Director of the Royal Observatory 

 of the Campidoglio, Rome. 



The vertical or zenith telescope is intended for the measurement of the 

 zenith distance of stars culminating near the zenith, with the use only of 

 the wire micrometer and mercurial horizon, without need either of inversion 

 or level. The telescope being directed towards the nadir, on the reflecting 

 horizon placed at a great distance below the object-glass, it is possible 

 simultaneously to determine the nadir or the prime vertical with a fixed equa- 

 torial thread, and to collimate, with the movable thread, the reflected image of 

 the stars that cross the meridian in the field of the telescope, and then to 

 measure the zenith meridian distance with the micrometric screw. In order 

 that the observation may be completely normal, it is necessary that the reflec- 

 tion of the stellar rays extend over the whole object-glass, which will be 

 effected when the distance D of the reflecting horizon from the object-glass of 



A 



the telescope satisfies the condition D = where A is the aperture of 



2 tan Z, 



the object-glass, and Z the zenith distance of the star. 



