I. MEASUREMENT. 343 



1760. Arabian Planisphere. Royal Museum in CasscL 



A small apparatus, of Arabian origin, remarkable for its great age and 

 comparatively good workmanship. It is in the form of a plate made of brass, 

 16 c.m. in diameter, provided with means for hanging up, and has a movable 

 alhidada. The front side shows a planisphere, the back various divisions and 

 tables covered everywhere with Arabian characters. On nine transposable 

 and one fixed plate can be shown 19 different planisphere drawings for 

 the same number of polar attitudes. 



1761. Combined Planisphere and Astrolabe. 



Royal Museum in CasseL 



A very strong plate of brass, 37*5 cm. in diameter, with an arrangement 

 for free suspension. On the one side is a planisphere with web, on the other 

 an astrolabe with double sight. The instrument dates from the 15th or 

 16th century. 



1762. Astronomical Circle, after Gemma Frisius, made in 

 1572, by Gualtherus Arssenius, grandson of Gemma Frisius. 



H. G. Van de Sande Bakhtiysen, Director of the Observa- 

 tory at Ley den. 



b. ALTAZIMUTH INSTRUMENTS. 



1763. 12-inch Altazimuth. Messrs. Trougliton $ 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. Altazimuth on Small Stand. L. Casella. 



1763b. Altitude and Azimuth Instrument, made by 

 Dollond with double altitude circles. 



W. Watson and Son, London. 



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

 telescope and the azimuth circle. The azimuth circle is an arrangement 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 smaller circle is divided to 10 minutes of arc, 

 and on the inner edge of larger circle are divided three derniers ; above these 

 is the large conical axis of azimuth circle, from the top of which spring rect- 

 angular 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 movements. Lamp with graduating aperture. 



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

 motion, constructed by Professor Kaiser. 



//. G. von Bakhuysen, Director of the Observatory, 

 Ley den. 



The stand possesses the advantage that the eye-piece, which is fitted with a 

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

 telescope, whilst the axis of the eye-piece remains horizontal. 



