I. MEASUREMENT. 395 



In. Table VI. is photographically represented the zenith telescope of 

 the Royal Observatory of the Campidoglio, constructed by Signer Ertel of 

 Monaco, from a design given by Professor Respighi. The telescope is 

 mounted as in transit instruments, and may also be used as a meridian 

 telescope ? the object-glass, made by Signor Merz, of Monaco, has a clear 

 aperture of 0* 108 m , with a focal distance 1 * 582 m . The micrometer is composed 

 of 1 1 fixed equatorial threads and two movable threads, with a fixed meridian 

 thread. The eye-piece can be moved with the greatest ease, whether in the 

 meridian or vertically. The micrometer can be turned 90 to render the 

 threads parallel to the meridian, and then the instrument serves as a transit- 

 instrument. 



1765. Four-inch Achromatic Telescope, on altazimuth 

 stand, with quick and slow motions, in altitude and azimuth. 



John Broivning. 



1768. Instrument for Easy Determination of Time by 

 Equal Altitudes of Different Stars. 



Colonel Zinger, Pulkowa. 



This instrument is constructed for easy application of the method developed 

 lately by Colonel Zinger for exact determination of time by equal altitudes of 

 different stars {see Viertelgebrschrift des Astronomischen Gesellschaft, 1875). 

 The principal condition is unaltered relation of level to telescope in going over 

 from a star in the east to another in the west by having found its vertical axis. 

 The divided circles serve only for setting the telescope. For using only bright 

 and well determined stars down to the (8 or 4) magnitude, 9 minutes of time 

 will be in the average sufficient to give time with the accuracy of 0' 1 s . The 

 eyepiece of the telescope is provided with a micrometer, to enable the 

 observer to get with the same instrument, by approximately equal altitudes of 

 two stars near the meridian, exact determination of latitude. 



c. TRANSIT CIRCLES AND QUADRANTS. 



1780. Model of the Greenwich Transit. 



South Kensington Museum. 



1767. Photograph of Meridian Circle, with object glass 

 of 6" aperture and 6' focal length, circle of 2' diameter ; under 

 construction for the Observatory of Strassburg. 



A. Rep sold and Sons, Hamburg. 



In this instrument the microscopes are attached to the heads of the cast- 

 iron columns, which also carry the bearings of the axis. One of the pivots 

 contains an objective (2" aperture), and the other a plate with a hole ; this 

 arrangement made at the suggestion of Prof. Winuecke, serves as a colli- 

 rnator for controlling the position of the axis. Perfectly central illumination 

 of the field of view is effected by a small mirror at the back of the objective. 

 The cells of the objective and eyepiece can be interchanged ; there is an 

 arrangement for Nadir observations, also for observations of the reflected 

 image of a star, with a Nadir distance of 8-60 by means of a movable 

 mercurial horizon. 



1768. Photograph of Transit Instrument, with azimuth 

 circle and a broken telescope of 3" aperture. 



A. Repsold and Sons, Hamburg. 



