PLATE 1. 



The Itumford spectroheliocrraph, attacbed to the forty-inch Yerkes refi'actor. 

 Tlie shaft which is driven by the declination motor may lie seen at the right. It 

 carries a f^rooved pulley near its lower end connected" with a similar pulley at 

 the end of the camera box bj' means of a round leather belt. On the same shaft 

 with this second pulley is a spur gear, which engages with the two gears on the 

 projecting ends of the screws that pass through the camera box. The keys used 

 to operate the split nuts that clamji the plate carriage to the screws, the windows 

 for observing the spectrum at the middle and at the ends of the second slit, and 

 the si-rcw-drivers employed to push forward the plate holder after the slide is 

 withdrawn, are on the top of the caniei-a box. At the left end of the box may 

 iie seen the door through which the i)late bolder is ins(>rted. and the narrow slid- 

 ing door in its outer face through which the slide is withdrawn, as well as the 

 micrometer heads of the screws for controlling the width of the second slit and 

 for moving it as a whole. The first slit, at the end of the collimator, is almost 

 hidden from view by the metallic screen required to shield its mounting from 

 the great heat of the solar image. Ijight reacb.es the first slit through a long 

 narrow opening in this screen. INIounted on four i)osts above the screen, at sucii 

 a height as to lie in the visual focal plane when the first slit is at the focus for 

 the K line, is a narrow metallic plate, on which a line is drawn in the direction 

 of disi)ersion. During an exposure, the limb of the Sun is m;ide to follow this 

 line. At the end of the electric cable may be seen the switches used for ()])erat- 

 ing the declination motor, and (just below) the rod with which the mirror in the 

 prism box is rotated. 



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