212 CARNEGIE INSTITUTION OF WASHINGTON. 



in which 6 is the width of the face of the octagonal mirror, d the diameter 

 of the concave mirror, r the distance of the image from the octagonal mirror, 

 and D the distance between the stations. The accuracy of the measurement 

 is directly proportional to r, so that a compromise must be made because of 

 rapid diminution of brightness with increase of r. A preliminary trial under 

 rather unfavorable conditions due to haze, unsteadiness of the air, insufficient 

 adjustment of the apparatus at the distant station, and low intensity of the 

 source of light, indicated that the return image would be sufficiently bright 

 for measurement under favorable conditions with a value r = 250 mm. This 

 would insure an order of accuracy of about 1 part in 50,000. If it is considered 

 practicable to place two, or better three, concave reflectors at the distant 

 station, the value of r could be nearly doubled, with a corresponding gain 

 in accuracy. 



More reliable data would have been obtained with the present arrange- 

 ment of apparatus, but that an exceptional amount of haze and smoke in the air 

 between the two stations, due in part to forest fires, has made observations 

 very difficult. Since the chances for immediate improvement did not appear 

 promising. Dr. Michelson returned to Chicago about August 15 with the in- 

 tention of resuming observations at Mount Wilson at a somewhat earlier 

 date next year. 



Through the cooperation of the Coast and Geodetic Survey, the measure- 

 ments have been completed for a very accurate determination of the distance 

 between the two stations used by Dr. Michelson for this investigation. The 

 work has been under the immediate direction of Mr. C. L. Garner, and the 

 reduction of the observations is being completed in Washington. 



EFFECT OF THE EARTH'S ROTATION ON THE VELOCITY OF LIGHT. 



Reference was made in the last annual report to the experiment devised 

 by Dr. Michelson to detect possible displacements of interference fringes 

 due to the rotation of the earth. Dr. Michelson has decided to use a pipe- 

 line exhausted of air for this work and will undertake the investigation in the 

 vicinity of Chicago, the necessary funds having been provided for the purpose. 



THE TORSION SEISMOMETER. 



As a member of the Advisory Committee on Seismology, Anderson has 

 assisted Dr. H. O. Wood in the development of seismometric instruments 

 for registering earthquakes of a local nature. A new type of instrument 

 called the torsion seismometer has been devised. Continuous records have 

 been made with one of these instruments since late in January 1923, with 

 results which are satisfactory in every way. A full account of the instru- 

 ment and its performance will be found in the annual report of the Advisory 

 Committee on Seismology. 



PREPARATIONS FOR THE SOLAR ECLIPSE OF SEPTEMBER 10, 1923. 

 Reference has already been made to the general plans for observing this 

 eclipse which have occupied for several months the attention of the members 

 of the staff as well as of the departments of design and instrument and optical 

 construction. These plans involve: first, direct photography of the corona 

 and the field of stars near the sun, together with photometric, radiometric, 

 and spectroscopic observations of the corona at a station near the center of the 

 path of the shadow at Point Loma, California; second, spectroscopic observa- 

 tions of the flash spectrum with slitless concave-grating spectrographs of 



