THE ABSOKPTIOTN- AiM]) KMISSION OF AI!{. 



in exposures to different jwrtions of tlie spectrum. At / is the veruier for llie 

 alidade of the prism ; but for a very fine setting this veruier is insufficient, and a 



Fig. 9. Horizontal Plan of Spf.ctrograi'ii. Half Natural Sue. 



niici 



ometer (not shown in tlie drawing) is connected with it, so that the angle 



may be read to two seconds of arc. This is used in the measurement of the 



