MOUNT WILSON OBSERVATORY. 



267 



5 per cent change of resistance upon exposure to the light of Arcturus 

 at the focus of the 60-inch telescope. The high resistance of the cell 

 (300 megohms) made it unsatisfactory, however, for use in series with a 

 galvanometer directly. It was therefore decided to attempt the use 

 of a thermionic vacuum tube for amplifying the galvanometer de- 

 flections. After some difficulty an electrostatic audion voltmeter was 

 perfected which was capable of measuring voltages as low as 10~^ volts. 

 The method of operation then consisted in connecting the thalofide 

 cell in series with a battery and a resistance the value of which was 

 the same as that of the cell. Changes in the resistance of the cell due 

 to exposure to light caused changes in the potential across the series 

 resistance. These potential variations were then measured with the 

 thermionic voltmeter. The first test of the device with the 60-inch 

 reflector gave the following results: 



Experiments are being carried on with a cell of smaller surface area, 

 with which it is hoped to reach much fainter stars. 



STELLAR SPECTROSCOPY. 



The stellar spectroscopic work of the year has been carried on by a 

 considerable number of observers, and their investigations have dealt 

 with a wide variety of problems. The following summary indicates 

 the principal divisions of the work: 



General program of radial velocities and determinations of absolute magnitude: Adams, 



Joy, Stromberg, Hoge. 

 Variables of type Md and stars with bright lines: Merrill, Humason. 

 Spectroscopic binaries and R-type stars: Sanford, Duncan. 

 Nebulous stars: Hubble. 

 Investigations of absolute magnitude for B and A stars: Lindblad. 



The two slit spectrographs have been in regular use at the Casse- 

 grain focus of the large reflectors, and a small spectrograph of short 

 focal length has been employed occasionally at the primary focus, both 

 with and without a slit. In addition, a considerable number of photo- 

 graphs have been obtained by Mr. Hubble, Mr. Humason, and Mr. 

 Lindblad with the 10-inch refractor and an objective prism. Most of 

 the spectrograms taken at the Cassegrain focus have been made with 

 a dispersion of one prism and a camera of 45 cm. focal length. The 

 18-cm. camera has proved most valuable, however, in the case of the 

 fainter variables which have been investigated. 



During the year, 1,549 spectrograms have been obtained with the 

 Cassegrain spectrographs, 634 with the 100-inch telescope, and 915 



