I. MEASUREMENT. 349 



1788. Equatorial, small, capable of carrying a telescope of 

 3 to 3 1 inch aperture with perfect steadiness. Ycatcs fy Sons. 



1789. Equatorial arrangement of a Refractor, of 9" aperture 

 and 13' focal length, with micrometer eye-piece. (Photograph.) 



A. Rcpsold and Sons, Hamburg. 



The declination circle can be read close to the eye-piece of the telescope ; 

 it is illuminated by the lamp which illuminates the threads in the field of 

 view of the telescope. By means of one perpendicular roller beneath the 

 centre of gravity, the pressure of the round axis on the bed-plate is removed. 

 The clockwork is arranged in the head of the cast-iron column of which the 

 first are below the floor. 



1790. Orbit-sweeper. Equatorial arrangement of a telescope 

 of 6" aperture, 8' focal length, with third axis, 1874. Constructed 

 for the Observatory of Strassburg. (Photograph.) 



A. Rcpsold and Sons Hamburg. 



In this equatorial arrangement the head of the declination axis has at 

 right angles to it the socket of a third axis, about which the telescope can be 

 revolved, whilst its optic axis can be inclined to the third axis at an angle of 

 90 + 2. By means of this arrangement a heavenly body is easily found 

 with a rapid movement of the instrument, as the third axis is directed to the 

 pole of the projection of its orbit. (See Airy's observations on an " Orbit- 

 sweeper," in the monthly notices of the E. Astron. Soc.) The telescope can 

 also be placed parallel to the third axis at the head of the declination axis, 

 and has then a simple equatorial movement. This instrument is provided 

 with clockwork, and has a changeable polar distance from to 66. 



1791. Model of the Great Melbourne Reflector, com- 

 pleted by Messrs. Grubb & Son in 1868. Scale, f inch to a 

 foot ( T V). Howard Grubb, F.R.A.S., Dublin. 



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 ., , , f 



Slow motion in AB - - f a Iable from eye end of telescope. 



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 Grubb, F.R.A.S., ^Dublin. 



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

 declination, is accomplished from eye end of great telescope. 

 Also quick motion in right ascension "| 



quick motion in declination - j 



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



slow motion in declination - j scope. 



clamping in right ascension - j 



,. clamping in declination - - j 



