MOUNT WILSON OBSERVATORY. 205 



for which readings on two successive nights showed a separation of 14 feet 

 between the two primary mirrors before the fringes disappeared. The 

 previous readings were 10 feet in 1920 and 8.5 feet in 1921. The star will be 

 observed carefully in an attempt to correlate these differences with variations 

 in light and radial velocity. 



Several stars of visual magnitude about 2.5 have been under observation 

 and the slight differences found in the visibility-curves indicate smaller 

 angular diameters for the stars of earlier spectral type. 



It is evident that any considerable advance in the measurement of stellar 

 diameters must await the completion of the 50-foot interferometer telescope 

 now under construction in the instrument shop. 



RADIOMETRIC OBSERVATIONS OF STARS. 



The measurement of stellar radiation with the vacuum thermo-couple has 

 been continued by Pettit and Nicholson as a regular part of the observing 

 program of the 100-inch reflector. During the year, 352 sets of observations 

 have been made and recorded with the photographic registering device, each 

 set consisting of 7 deflections on total radiation and 7 deflections through the 

 1-cm. water-cell. The observations on each night include three or more 

 measurements on two stars, one early and the other late in the night, over 

 an interval during which the air-mass for each changes about threefold. 

 These are used for the determination of the absorption coefficients of the 

 atmosphere for the night. 



Considerable attention has been given to long-period variable stars, 22 

 of which have been observed. Since March 1922, R Leonis has been observed 

 on 14 nights, R Aquilae on 11, and o Ceti on 12. The faintest star of this 

 class so far studied is x Cj^gni, which was of the twelfth magnitude on May 

 17, 1923. Its total radiation on that date was' equal to that of Regulus, a 

 star of type B8 and magnitude 1.3. The variable, accordingly, was radi- 

 ating 19,000 times as much energy as an AO star of the same magnitude. On 

 September 24, 1922, the star had a magnitude of 4.3 and gave a deflection but 

 slightly greater than that from Altair, or only about 230 times the radiation 

 of an AO star of equal magnitude. For a change in light of 1,320 times, the 

 total radiation changed but 1.7 times, thus indicating the rapid shift of 

 energy into the visible spectrum as the variables of this class approach their 

 maxima of light. 



Observations of the Cepheid variables 5 Cephei and tj Aquilae show an 

 absorption by the water-cell of about 0.4 magnitude at maximum of light 

 and 0.6 at minimum. These values correspond to those given by an FO 

 star at maximum and an early G star at minimum, results which are in 

 close agreement with the spectroscopic observations. 



Nova Aquilffi of 1918 was observed on September 25, 1922. The water-cell 

 absorption amounted to 0.2 magnitude, which corresponds to that of a star 

 of an early B-type. 



A small spectrograph has been constructed for obtaining stellar spectral 

 energy curves with the thermo-couple. A preliminary trial of the instru- 

 ment at the Newtonian focus of the 100-inch reflector has given satisfactory 

 photographic curves for Arcturus and Antares. An eyepiece containing a 

 cell filled with a solution of neodymium chloride is used for observations of the 

 spectrum as it traverses the thermo-couple junction, thus making it possible 

 to determine reference points in the visible spectrum. 



