MOUNT WILSON OBSERVATORY. 207 



sodium, and potassium, preliminary measures indicate that the variation of 

 conductivity with the square root of the concentration appears to hold for 

 temperatures as high as 1600° C. The use of temperatures as high as 2000° 

 seems to be well within the range of the apparatus. 



The value of Anderson's method of electrically exploded wires has been 

 further demonstrated. A sensitive thermopile, used in conjunction with a 

 quartz spectrograph, has permitted the spectral energy distribution of the 

 radiation to be measured, showing that the maximum value of EX is at a 

 wave-length less than X2800 and probably below X2400. The energy dis- 

 tribution con-esponds almost exactly with that of a black body at a tem- 

 perature of 20,000° C. Although the average pressure in the explosions is less 

 than 5 atmospheres, the layer of vapor (about 4 cm. thick) is completely 

 opaque to radiation. This fact may prove to be of considerable importance in 

 solar and stellar physics (p. 248). 



Babcock has made an interesting study of the green auroral line with a 

 Fabry-P^rot interferometer. With a camera working at F/4 or faster, the 

 fringes can be photographed in a few hours at Pasadena on any night, even when 

 the sky is moderately cloudy. PreUminary work on Mount Wilson shows that 

 the width of the hne does not exceed 0.035 a. Improved instrumental arrange- 

 ments will soon give precise measures of the wave-length and a reliable deter- 

 mination of the maximum width of the line (p. 240). 



At the desire of Dr. Day, director of the Geophysical Laboratory of the 

 Carnegie Institution of Washington, a cooperative plan has been undertaken 

 for the study of the volcanic gases emitted by the crater of Kilauea at times 

 when the level of the lava is high. Attempts will be made to photograph the 

 spectra of the burning gases with small spectrographs especially built for the 

 purpose in our instrument shop. These were designed by Babcock, who will 

 join the expedition when conditions at the volcano become favorable for 

 observational work (p. 253). 



Reference was made last year to the repetition on Mount Wilson of the 

 Michelson-Morley experiment by Professor Dayton C. Miller, of the Case 

 School of Applied Science. Although this work has been carried on as an 

 independent investigation, we are permitted by Professor Miller to give the 

 results of his most recent observations. These indicate that the displacement 

 in the direction of that due to a possible ether drift can not exceed one-tenth 

 of the predicted amount. It is possible, however, that certain factors to be 

 considered in computing the predicted displacement are not complete 

 (p. 245). 



Dr. H. O. Wood has continued his seismological investigations under the 

 auspices of the Carnegie Institution of Washington. The Observatory has 

 been able to render assistance to a certain extent in the design and con- 

 struction of apparatus. 



The constant search for new instruments and methods, always a prime 

 factor in our pohcy, has been continued throughout the year. The chief 

 results, most of which have been mentioned in preceding paragraphs, are the 

 method of measuring the absolute magnitudes of the A stars ; the appHcation 

 of the bolometer to the measurement of the distribution of energy in stellar 

 spectra; the improvements in thermo-couples and their appUcation to the 

 Koch microphotometer; the use of the electric furnace for precise conductivity 



