350 SEC. 11. ASTRONOMY. 



The one lamp hanging in end of declination axis illuminates 



Upper right ascension circle. 



Decimation circle in 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 1 2 feet 

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



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 on the head of the declination axis. The scales 

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

 micrometer is close to the eye-piece of the telescope, and the same magnifier 

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

 slide bars of the objective move simultaneously on cylindrical surfaces in 

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

 made close to the eye-piece. 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 Dusseldorf. Carl Bomber g, Berlin. 



c. EYE-PIECES. 



1795. Eye-piece Shutter for Telescopes. Allowing the 

 aperture to be opened and closed by turning the head of the eye- 

 piece. Captain J. E. Davis, R.N., F.R.G.S. 



This is effected by fitting the kidney-piece with a fulcrum pin and a lever, 

 the latter passing through the side, which is acted on by the head being 

 turned. It obviates the necessity of the slide or kidney-piece fitted with a 

 protruding pin, the latter frequently breaking the nail, or (with gloves on) not 

 being felt ; the pin also often loosens, and drops out. 



1796. Eye-piece Heliometer. 



C. A. Steinhtil, Sons, Munich. 



In this instrument the images are formed by means of two rectangular 

 prisms, each of which revolves on an axis giving measurements by a screw 

 micrometer. The prisms reflect at less than 45, and are placed in parallel 

 lines of light. Thus when the reflecting surfaces form an angle with each 

 other, the pencils of rays do not issue mutually distorted, as in other helio- 

 meters, but remain centrical at all angles ; also the varying distance of 

 the -mirror, and of the plan of the image has no longer an influence on the 

 excellence of the image, which appears without parallax. A small telescope 

 with objective prisms serves as eye-piece ; it is placed parallel to the telescope 

 axis. The mutual illumination of the image changes with the distance of the 

 greatest diameter from the field of view, in which the images respectively are 

 movable. Any illumination which is taken away from one image is added to 

 the other ; the position-circle gives single minutes. 



