39 



moon, is passed through a prism, and the resulting 

 series of bright lines is recorded upon a sensitive 

 plate. But the study of this 'flash' spectrum 

 has been seriously hampered by its momentary 

 visibility, occurring only at intervals of years. With 

 the 6o-foot tower telescope the numerous bright 

 lines of the 'flash" can be photographed on any 

 day of good definition, with a spectrograph more 

 powerful than those used in eclipse observations. 

 In this way we not only get a marked increase in the 

 accuracy of measuring these lines and their dis- 

 placements, but we also find it possible to study 

 the phenomena of levels lower than are attainable 

 at eclipses. Some remarkable modifications of the 

 dark-line solar spectrum at the sun's limb have also 

 been found on these photographs. 



THE 1 5O-FOOT TOWER TELESCOPE. 



The success of the first tower telescope indicated 

 that the construction of a more powerful instru- 

 ment, giving a larger image of the sun (16 inches 

 in diameter), would be fully warranted. To secure 

 the necessary steadiness of mirrors and lenses 

 mounted 160 feet above the ground, the plan was 

 adopted of incasing each steel member (leg or 

 cross-bracing) of a skeleton tower within the 

 corresponding hollow member of another skele- 

 ton tower, with sufficient clearance to prevent 

 contact. The inner tower thus carries the instru- 

 ments; the outer tower carries the dome to cover 

 them, while its members serve as an efficient 



