MOUNT WILSON OBSERVATORY. 263 



The preliminary work was done with a condenser of 0.4 microfarad 

 capacity, charged from a 26,000- volt transformer, the current from the 

 secondary of the transformer being rectified by a mechanical device. 

 Later a condenser of approximately 1.0 microfarad was constructed 

 and used with the same transformer. With the larger condenser the 

 brightness of the explosion was increased very much in the region of 

 longer wave-lengths. There did not appear to be a corresponding 

 increase in the region of short wave-lengths, suggesting that as the 

 amount of energy is increased, the source becomes redder. 



The work will be continued, using higher voltage, and, if possible, 

 more energy in the discharge. It is also planned to study the explosions 

 in gases other than ak, such as hydrogen and perhaps helium. Since 

 this method makes it possible to produce in the laboratorj^ spectra 

 similar to those of the sun and the stars, it is likely to be of considerable 

 importance in astrophysical investigations. 



AIR LINES IN SPARK SPECTRA. 



Since the last report some additional work has been done by Mr. 

 Merrill on the air lines given by metallic sparks in the red and infra-red. 

 By operating the spark in an atmosphere of oxygen, several identifica- 

 tions of oxygen and nitrogen lines by previous observers using vacuum- 

 tubes were confirmed, and the doubtful identification of X7157 with 

 oxygen was strengthened. 



DISCHARGES IN HIGH VACUA. 



Miss Carter has resumed experiments on the spectra of discharges 

 in high vacua, using now the brief, arc-hght discharge which takes place 

 between metallic electrodes when a high potential, with condenser, 

 breaks down the resistance of the vacuum. The characteristics of 

 such spectra are little known. 



Using a source of this type to produce lines of very short wave- 

 length, Professor Millikan observed a notable strength in the extreme 

 ultra-violet. This result made it desirable to examine the range of 

 spectrum usually observed, and Miss Carter has now completed the 

 observations for the spectra of iron, titanium, calcium, cadmium, and 

 magnesium. The lines obtained greatly exceed in number those re- 

 corded in previous observations with the high-vacuum spark. A 

 detailed study has not yet been made, but the spectra resemble closely 

 the regular spark spectrum, except in the case of iron, for which a 

 similarity to the spectrum of the core of the arc seems to prevail. 



CONSTRUCTION DIVISION. 

 DRAFTING AND DESIGN. 



The chief work of the drafting department during the year has 

 been the design of a 20-foot stellar interferometer and compensator; an 

 observing platform for the 134-foot Cassegrain focus of the Hooker 



