210 CARNEGIE INSTITUTION OF WASHINGTON. 



two independent sets of fringes, one due to each star. On rotating the 

 disk carrying the sUts, the visibiUty of the fringes is seen to vary with 

 the position angle. When the Hne joining the sUts corresponds with the 

 Hne joining the stars, the sHts are separated until the fringes of one set 

 fall exactly between those of the other, causing minimum visibility, or 

 complete disappearance if the two stars are of equal brightness. The 

 distance between the slits then permits the angular distance between 

 the two stars to be computed with a precision greatly superior to that 

 attained in the micrometric measurement of wide binaries. As an 

 indication of this precision, it may be added that the greatest difference 

 between the observed and calculated positions of the components of 

 Capella, as determined by Mr. Anderson, is four hundred-thousandths 

 of a second of arc. An account of the method describing the technique 

 developed by Mr. Anderson for the measurement of both position angle 

 and distance may be found on page 252. It is hoped that through a 

 cooperative plan of observation, in which other observatories will take 

 part, a large number of close binaries may be measured by the inter- 

 ferometer. 



The simple use of two slits permits the theoretical resolving power of 

 the Hooker telescope to be more than doubled under ordinary atmos- 

 pheric conditions, and this result can be still further improved by at- 

 taching a large interferometer to the upper end of the telescope tube. 

 An instrument of this sort, in which two plane mirrors, used instead of 

 slits in combination with the 100-inch mirror, can be separated to a 

 distance of 20 feet, has been built in our shop and tested on stars by 

 Professor Michelson. The sharpness of the fringes observed with this 

 enormous resolving power, which corresponds with the theoretical 

 resolution of a telescope of about 40 feet aperture, indicates that atmos- 

 pheric difficulties would not be likely to stand in the way of a much 

 greater extension of the method. It remains to be seen whether it will 

 prove feasible to accomplish Professor Michelson's desu-e to measure 

 the angular diameter of a star (p. 250). 



The truly extraordinary precision of measurement attained in the 

 case of Capella naturally raises the question whether a comparable 

 advance can be accomplished in the measurement of stars several min- 

 utes of arc apart. If this were feasible, the determination of parallaxes 

 and proper motions might be greatly improved, and it might become 

 possible to measure the displacement of a star caused by the gravita- 

 tional field of Jupiter (Einstein effect). Professor Michelson has de- 

 signed several forms of interferometer for this purpose, and one of 

 them will soon be tested with the Hooker telescope. It is feared, 

 however, that atmospheric disturbances, which would differ along the 

 optical paths of the stars several minutes apart, may prevent the fringes 

 from being observed. 



