250 



CARNEGIE INSTITUTION OF WASHINGTON. 



(5) A number of observations of stellar spectra with the 10-inch 

 reflecting telescope and an objective prism have been made by Mr. 

 Humason and Mr. Merrill, mainly in the Ha region of the spectrum. 

 About 10 or 12 stars have been found to have a bright Ha line for which 

 no bright lines were known previously. A remarkable group of objects 

 with bright Imes has been found by Mr. Humason near the position 

 R. A. 17^ 40"*, Dec. —32°, in addition to others aheady known in 

 this region. 



(6) The following objects were observed for radial velocity by Mr. 

 Sanford with a small spectrograph at the primary focus of the 100-inch 

 telescope: 



STELLAR INTERFEROMETER. 



As set forth by Professor Michelson in the Astrophysical Journal, 

 vol. 51, 257-262, 1920, Mount Wilson Contribution No. 184, one of the 

 lines of investigation which seemed sufficiently promising to warrant a 

 preliminary trial was the measurement of the size of stars by the inter- 

 ference method. As shown in this article, the possibility of such an 

 application of the interferometer, aside from mechanical difficulties 

 (some of which proved not entirely negligible) is determined by the 

 effect of atmospheric disturbances on the appearance of the inter- 

 ference fringes. 



The results of actual trials had shown not only that interference 

 fringes were clearly visible with a base-line of the full aperture of the 

 40-inch refractor at Yerkes Observatoiy, of the 60-inch reflector and 

 the 100-inch reflector at Mount Wilson, but also that exceedingly 

 accurate measurements (such as the determination of the separation of 

 close double stars, etc.) could be made even when the seeing was so 

 poor that the usual observations were impossible. The application 

 of the method to the measurement of the system of Capella is described 

 below. 



In explanation of so unexpected a result, it may be suggested that 

 in bad seeing, when using the whole aperture of the objective, there is an 

 integi-ated efl"ect of the light-waves meeting in all possible phases, which 

 tends to obliterate the details of the diffraction-pattern of the star- 

 image, but that when two light-pencils are selected at opposite ends 

 of a diameter, the result is not an integration, but a mere displacement 

 of the diffraction-pattern, sufficiently small for the eye to follow. Such 



