Gtrubb — A Heliostat for the Smithsonian Institution. 601 



spectroscope placed at 50 ft. distance, the slit of which is always in 

 the same position, and of course vertical. 



The slow motions in right ascension and declination are avail- 

 able to the observer at this 50 ft. distance by cords, &c. ; but, 

 besides, there are other motions necessary. 



As the slit is necessarily always vertical, it is sometimes desir- 

 able to be able to give a purely vertical or horizontal motion to 

 the image. When the object is exactly on the meridian, the slow 

 motions in right ascension and declination do then give horizontal 

 and vertical motions, but not when in any position off the meridian. 

 It is necessary, therefore, to have another pair of slow motions 

 which will cause the image to move in a purely vertical and 

 horizontal direction, whether the object be at the meridian or off 

 of it. 



The vertical slow motion is effected by the simultaneous raising 

 or lowering of the two vertical bars whioh carry the bearings of 

 the mirror trunnions. On the upper end, each of these bars has a 

 screw cut upon it ; the nuts which fit these screws are cut into 

 teeth, forming mitre wheels, and these two mitre wheels or nuts 

 are coupled together by a horizontal shaft to which the 50-ft. slow 

 motion rod is geared. 



The horizontal motion is effected by a tangent screw and nut 

 which moves the whole mirror frame horizontally about a centre, 

 which is vertically under the centre of the declination axis. In 

 addition to the small horizontal motion required for this, the whole 

 mirror frame can be rotated about this same centre through 45° 

 on each side of the meridian, so that for certain special purposes 

 the reflected beam of light can be directed to the S.E. or S.W. 

 instead of due S. The vertical and horizontal slow motions men- 

 tioned above are connected to shafts 50 ft. long to bring them 

 under the control of the observer. 



The adjustments of these instruments are very numerous and 

 delicate. It may be interesting to note how some of these were 

 effected. 



A portion of the " bushings " carrying the trunnions of the 

 piece which slides on the bar attached to the mirror was cut away 

 to allow a striding or hanging level to be applied to these 

 trunnions. 



By a series of testings and reversals, these trunnions were 



