402 SEC. 11. ASTRONOMY. 



e. 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. 



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 being fitted 

 with a protruding pin, which frequently is the cause of breaking the nail, or 

 (with gloves on) not being felt ; the pin also often loosens, and drops out. 



1796. Double-Image Micrometer or Eye-piece Helio- 

 meter. C* A. Steinheil 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 mi- 

 crometer screw. The prisms reflect at an angle of 45, and are placed in a 

 pencil of parallel rays 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 heliometers, but remain central at all angles ; also the var} r ing distance 

 of the mirror from the plane of the image has no longer an influence on the 

 definition of the images, which appear without parallax. A small telescope 

 with its objective parallel to the axis of the telescope serves as eye-piece ; it is 

 placed parallel to the telescope axis. The relative illumination of the images 

 changes with the distance of the greatest diameter from the field of view, in 

 which the images respectively move. Any illumination which is taken away 

 from one image is added to the other ; the position-circle gives single minutes. 



1796a. Zollner Eye-piece, to be used as a star spectroscope 

 for very small stars, and giving especially good definition. 



A. Hilger. 



f. MICROMETERS. 



1797. Collection of the finest Micrometer Screws, of 



almost perfect accuracy, and a small instrument for observation. 



Hugo Schroder, Hamburg. 



This apparatus serves for the examination of micrometer screws ; those 

 exhibited are shown as examples of the great accuracy and delicacy which 

 can be attained by cutting the screws according to the method invented by 

 Hugo Schroder. A table showing the results of the examination of one of 

 the screws by Dr. Vogel is subjoined. 



1797a. Collection of Micrometer Screws, cut with 

 Hilger's special apparatus, for astronomical purposes. 

 40 threads to the inch (1). 

 100 (2). 



200 (1). 



1,000 (1). 



A. Hilger. 



1798. Position Micrometer, constructed for the refractor 

 of the Royal Observatory at Berlin. Carl Bamberg, Berlin. 



