43 



graph, of which it is the direct successor, the 75- 

 ioot spectrograph gives results fully 20 times as 

 accurate. 



FIG. 33. Observing Room and 75-foot Spectrograph. 

 THE SUN AS A MAGNET. 



What this means in practice is illustrated by a 

 recent investigation. As already explained, many 

 of the spectrum lines are split into two or more 

 components by a magnetic field. A Nicol prism 

 and quarter-wave mica plate, placed over the slit 

 of the spectrograph, permit us in the laboratory 

 to cut off either component at will. The use of 

 a compound quarter-wave plate, made of narrow 

 strips, gives the serrated appearance of the lines 

 shown in Fig. 22. If the lines in the solar 



