PLATE I. 



The liiimfovfl spectrohelioa;raph. attached to the forty-inch Yerkes refractor. 

 The shaft which is driven by the decHnatioii motor may he seen at the riglit. It 

 carries a jirooved i)ulley near its h>\ver end connected witli a simihir pulley at 

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

 with this second imlley is a spur .m-ar, which eni,'a,i::es 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 clami) 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 screw-drivers employed to push forward the idate holder after the slide is 

 withdrawn, are on tl«^ top of the camera box. At the left end of the box may 

 Pe seen the door through which the plate holder is inserted, 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 ing door in its outer face through which the slide is withdrawn, as well as the 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 ;ilmost 

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

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

 narrow opening in this screen. Mounted on four posts above the screen, at such 

 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 dispersion. During an exi)osure, the limb of the .Sun is made to follow this 

 line. At the end of tlie electric cable may be seen the switches used for operat- 

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

 l)rism box is rotated. 



146 



