8 SECTION-PHYSICS. 



motion given to it by a driving clock, or by giving suitable motions by 

 clockwork to a large plane mirror placed in front of the telescope, 

 which may then be immoveable,so that the light of the star as it travels 

 from east to west will be always reflected in the same direction, and 

 the star's image consequently remain stationary at one place. This 

 latter instrument, called a heliostat or sidereostat according as it is 

 applied to the sun or to the stars, is well represented in this collection 

 by a beautiful apparatus designed by Colonel Campbell and constructed 

 for him by Mr. Hilger. 



When by either of these methods we have obtained a brilliant and 

 stationary image of the heavenly body, it then becomes necessary to 

 interpose somewhere in the path of the light, a prism or prisms by 

 which it may be separated into the different kinds of light which come 

 to us bound up together in the star's light. A prism of small angle 

 may be placed before the object-glass of the telescope. This plan 

 possesses some advantages, but the size of the telescope is limited by 

 the size that can be given to the prism. The other and more usual 

 method consists in subjecting the light to the separating power of the 

 prism after it has been condensed by the object-glass or mirror. If a 

 cursory general examination of the spectra of stars only (the images of 

 which are points of light) is required, a very excellent form of apparatus 

 consists in placing a direct vision prism immediately in front of the 

 field lens of a negative eye-piece. When the eye-piece is withdrawn 

 for a short distance, the spectrum is seen well defined and of sufficient 

 width without the use of a cylindrical lens. 



If measures, and especially comparisons with terrestrial spectra are 

 desired, and if the objects, as the nebulae and planets, have images of 

 sensible size, it is best to employ a complete spectroscope. Several 

 forms of this instrument by Mr. Browning are in the exhibition. The 

 slit of the instrument is placed exactly at the spot where the image of 

 the heavenly body is formed. The diverging rays are brought parallel 

 in the usual way and the spectrum is viewed in the small telescope 

 of the instrument. 



Having by one of the above methods obtained the spectrum of a 

 heavenly body, we have to seek the best method of applying exact 

 measurement to the spectrum. This may be accomplished by some 

 "orm of micrometer, or by simultaneous comparison with a known 



