4 2 



beneath the tower. With this powerful instrument 

 the D lines of sodium, \vhich are barely separated 

 with the standard one-prism spectroscope of the 

 chemist, are about 1.2 inches apart in the third- 

 order spectrum. A photograph of the solar spec- 

 trum on this scale, including the ultra-violet but ex- 







_ . -- , 



rf- --- f 



L _ - , - --- y i 





. 



'-. 





: 1 







FIG. 32. Summit of i5O-foot Tower Telescope. 



eluding the infra-red, would be 70 feet long. With 

 the aid of Koch's recording microphotometer, the 

 gain in precision of measurement is directly pro- 

 portional to the length of the spectrograph. Thus, 

 as compared with the 3.5 foot Kemvood spectro- 



