I. ME ASUREM ENT. 40 1 



Diameter of great mirror, 48 inches. 



Focus, 30 feet 6 inches. 



Form, Cassegrainian. 



The ventilated tube formed of steel lattice bars. 



Quick motion in declination""! 



Slow motion in declination I ftvailable from e end of telescope. 



Slow motion in KA - - j 



Clamping in declination - J 



1792. Model of the Great Refracting Telescope, of 27 



inches aperture, for the new Imperial Observatory at Vienna, now 

 in course of construction at Mr. Howard Grubb's new Astronomi- 

 cal Works, Rathmines, Dublin. Scale, 1 inch to a foot ( T Vth). 



Howard Gi>ubb, F.R.A.S. 



In this instrument the reading of all circles, right ascension as well as 

 declination, is accomplished from eye end of great telescope. 

 Also quick motion in right ascension 

 quick motion in declination 

 slow motion in right ascension - I All available from eye end of tele- 

 slow motion in declination - f scope. 

 clamping in right ascension - ! 

 ,. clamping in declination - - J 



The one lamp hanging in end of declination axis illuminates- 

 Upper right ascension circle. 

 Declination circle on two opposite sides. 

 Bright and dark fields of micrometer. 

 Position circle of micrometer. 

 Field of 4-inch finder. 



A second right ascension circle is available for reading from ground floor 

 (south end), where also is a handle for quick setting, right ascension, and a 

 sidereal clock face. The base of the instrument forms a chamber about 12 feet 

 by 4| feet, in which is contained the clock. 



1792a. Model of Equatorial Mounting for the 3-ft. 

 Reflector at Parsonstown, recently erected for the Earl of 

 Eosse. Earl of Rosse, F.R.S. 



1793. Photograph of a Heliometer, with object glass of 

 4" aperture and 5' focal length. This instrument was used by the 

 Russian expedition for the observation of the transit of Venus, 

 1874. A. Repsold and Sons, Hamburg. 



The telescope revolves in the head of the declination axis. The scales 

 on both halves of the objective can be read by one microscope, of which the 

 micrometer is close to the eyepiece of the telescope, and the same micro- 

 scope serves to read the metal thermometer on the head of the objective. 

 The slides of the objective move simultaneously oh cylindrical surfaces in 

 opposite directions. The position circle can be read and all the movements 

 made close to the eyepiece. The instrument is mounted equatorially with 

 changeable polar distance from to 66, and is moved by clockwork. 



1794. Photograph of an Equatorial Refractor constructed 

 for the Observatory in Diisseldorf. Carl Bamberg, Berlin. 



40075. C c 



