Gtrubb — A Heliostat for the Smithsonian Institution. 599 



The simplicity of this arrangement is so great that it would no 

 doubt be invariably adopted were it not that the second reflection 

 causes some additional loss of light, which, for some purposes of 

 research, is undesirable. 



The single mirror heliostat is a much more complicated instru- 

 ment. The best known arrangement is that called the Foucault 

 heliostat. There are many modifications to suit particular condi- 

 tions, notably that of Dr. Stoney's, which has already, I think, 

 been described at' this Society, but the same general principle is 

 embodied in all. 



The single mirror heliostat consists essentially of two parts. 

 One is a mirrorlmounted with a vertical and horizontal motion, 

 and capable, therefore, of being placed in any angular position. 

 It is carefully balanced, and attached to its box is a strong cylin- 

 drical bar. In Foucault's design this bar projects at right angles 

 to the plane of the mirror ; in Dr. Stoney's the bar is in the same 

 plane, or parallel to the plane of the mirror. 



The second portion of the instrument is similar to any ordinary 

 equatorial mounting of a telescope. It has its polar axis set 

 parallel to the pole of our earth, and carried by clockwork : on one 

 end of this is mounted a declination axis, which generally carries a 

 small telescope for finding purposes. Across this declination axis 

 is mounted another, which is prolonged into a fork-shaped casting 

 at one end, in which is carried on a pair of trunnions a casting 

 which slides on to^the bar above described as attached to the mirror 

 box, and which forms the connection between the two parts of the 

 apparatus. 



If the equatorial portion of the instrument be accurately 

 adjusted and placed in position, and the distance between the 

 centre of declination] axis^and the centre of motion of mirror be 

 made exactly equal to the distance between the centre of declina- 

 tion axis and the centres^on which the slipping pieces work in the 

 fork, any object seen in the little finder telescope is reflected from 

 the mirror in a direction which is a prolongation of a line drawn 

 from centre of declination axis to centre of motion of mirror. If, 

 then, the polar axis be kept rotating at the proper speed and in the 

 proper direction, the light from that object will be reflected in a 

 constant direction. 



To anyone who has used a single mirror heliostat, or is familiar 



