2O 



cut off by the moon. The spectroheliograph not 

 only permits the prominences to be photographed 

 on any clear day, but discloses extensive clouds of 

 calcium, hydrogen, iron, and other vapors, which 

 do not rise high enough to be observed in elevation 

 at the limb, but are recorded (as flocculi) in pro- 

 jection against the bright disk. To the eye at the 

 telescope, or in direct photographs of the ordinary 



FIG. 13. The Chromosphere photographed without an Eclipse. 



kind, these flocculi are wholly invisible. The spec- 

 troheliograph brings them to view by excluding 

 from the photographic plate all light except that 

 due to calcium or hydrogen, as the case may be. 

 The measurement of these plates with the helio- 

 micrometer (Fig. 55), an instrument devised and 

 constructed in our instrument-shop, gave directly 

 the latitudes and longitudes of the flocculi, without 



