252 CARNEGIE INSTITUTION OF WASHINGTON. 



with his photographic work on nebulae and their spectra and on the 

 spectra of star clusters and Novse. Dr. Arnold Kohlschtitter, who was 

 captured by the British while on his way to join the German army, is 

 held in England as a prisoner of war. Dr. Harlow Shapley, in addition 

 to other work in stellar photometry, has made an extensive study of the 

 colors and magnitudes of stars in Messier 13. Dr. Adriaan van 

 Maanen has devoted all of his observing time to work on stellar paral- 

 laxes and has continued his measurements of spectra showing the gen- 

 eral magnetic field of the sun. Dr. Walter Colby, who came from the 

 University of Michigan in October to spend a year at the Observatory, 

 has shared in the solar observations with the Snow and tower telescopes 

 and in the work with the Koch microphotometer. ]\Ir. R. S. Capon, of 

 Oxford, who had been engaged in various kinds of solar work, returned 

 to England in December and is now serving in the army. Mr. George 

 P. Luckey, who began work at the Observatory in February, has 

 assisted in the solar and stellar spectroscopic observations and devoted 

 much time to vortex experiments. Mr. George S. Monk has taken 

 part in the solar and stellar spectroscopic observations and in the work 

 with the microphotometer. 



Professor J. C. Kapteyn has continued his investigations as Research 

 Associate. In anticipation of remaining in Holland this summer, he 

 doubled the time of his stay on Mount Wilson last year, and hopes to 

 return as usual in June 1916. Professor Carl Stormer, who has com- 

 pleted for pubUcation his investigation of the electromagnetic theory of 

 the hydrogen floccuH, has been reappointed as Research Associate. 

 Professor T. C. Chamberhn, of the University of Chicago, who has 

 long been a Research Associate of the Carnegie Institution of Washing- 

 ton, spent two months on Mount Wilson during the suimner of 1915. 

 During this time he prepared for us an extended paper on a "Tentative 

 Interpretation of Nebulae," which will be published in the Contributions. 



Director C. G. Abbot, of the Smithsonian Astrophysical Observatory, 

 has continued his work on Mount Wilson during the summer of 1915 

 with the assistance of Mr. L. B. Aldrich. 



INVESTIGATIONS IN PROGRESS. 



SOLAR RESEARCH. 

 DIFFERENTIAL METHODS OF MEASUREMENT. 



The recent rise into significance of the fourth decimal place in 

 wave-length measurements of spectrum hues is one of the most impor- 

 tant developments of modern spectroscopy While it promises a long 

 series of discoveries in the future, it greatly compUcates the work of 

 the present. In the electric arc, as the investigations of Mr. St. John 

 and Mr. Babcock have recently shown (see p. 278), displacements of 

 several thousandths of an angstrom occur for many lines in passing 

 from the center to the pole. Such shifts, which exhibit large variations 



