in Theory and Practice. ?>lb 



this is a balcony for the heliostat. The beams carrying the 

 instrument are placed about eight feet from the floor, and a 

 platform erected at one end of the room, thus allowing the 

 floor-space to be used for other purposes if necessary. 



AB and AC (see figs. XII., XIII.) are heavy wooden beams 

 6 X 13 in. and 23 feet long. AB is fastened rigidly to the 

 wall, while AC has a slight freedom of rotation about A, 

 controlled by screws at C The "ways^^ for the grating- 

 holder and camera-box are fastened to these beams by screws 

 which admit of adjustment, so that the "ways" may be 

 straightened if the beams warp. They are made of ^-inch 

 angle-iron, although a board made of any hard wood may be 

 used. G G' is a 4 in. tubular wrought-iron girder, braced by 

 a truss, and pivoted at its ends, directly over the " ways," on 

 two iron carriages. Its length is approximately equal to the 

 radius of the grating, and has a range of adjustment of about 

 six inches. The carriages have each two brass wheels or 

 rollers placed nearly a foot and a half apart, and these resting 

 on the iron ways enable the girder to be easily moved from 

 one position to another. The camera-box and grating-holder 

 are themselves movable along BC and have freedom to revolve 

 around axes, but can be finally clamped in place. The camera- 

 box (see fig. XIY.) consists of a fixed wooden frame B, and a 

 box A which can be removed. The sensitive plate is placed 

 in A in suitable slots and is pressed firmly by means of wooden 

 buttons against pieces of hard rubber so that it is bent to the 

 proper radius. There is in B a frame which can be moved 

 vertically by a rack and pinion ; and to this A is fastened by 

 dowel pins on the bottom and hooks at the top. On the back 

 of the camera-box, B, is hinged a board " C,'" which can be 

 held firmly in place by hooks. This board carries a brass 

 plate (see fig. XY.) having a longitudinal opening of a width 

 equal to the thickness of the plate and capable of revolution 

 around a horizontal central axis. By means of stops this 

 revolution is confined to 90°. This plate is used for the com- 

 parison of spectra, as described below. 



The grating-holder is made of brass. It consists (see 

 fig. XVI.) of a heavy platform carrying an upright frame, B, 

 which can move in slots on A. To B is fastened by screws 

 at the sides, P, a square piece of brass D. D is movable 

 around the axes, P, by means of a screw S. To D by means 

 of an axis P', at the bottom is fastened the frame, C. By 

 means of a screw at S' whose nut is rigidly connected with 

 D, C can be moved around the axis, P'. Springs take up the 

 slack of the screws when unscrewed. 



The grating itself stands on two projections at the bottom 



