254 CARNEGIE INSTITUTION OF WASHINGTON. 



With a view to testing the feasibiUty of the method, and especially 

 to investigate the intensity and the sharpness of the return image, a 

 preliminary trial was made, using the roof of the Chemical Laboratory 

 of the Cahfornia Institute of Technology for the home station and a 

 pier erected at the base of Mount Lowe for the distant station, the dis- 

 tance being a little over 4 miles. The result was very favorable as 

 regards intensity, but the seeing was never satisfactory, so that the 

 return image was not sufficiently sharp to measure with the required 

 degree of accuracy. 



A second preliminary trial is now in progress, with Mount Wilson as 

 the home station and the Puente Hills for the distant mirror, the dis- 

 tance being something over 16 miles. It is hoped that the increased 

 elevation will give a much clearer atmosphere, with greater steadiness 

 and sharpness of the return image. 



It may be noted that in the first trial the return image was viewed 

 upon a fairly bright background caused by the diffuse light due to the 

 direct illumination of the revolving mirror. In the present trial the 

 light is returned to the opposite face of the mirror, which is entirely 

 dark, thus increasing very considerably the visibility of the return 

 image. It may also be mentioned that an incidental advantage of the 

 octagonal form of the revolving mirror is an eight-fold intensity over 

 that from a single face. 



PROFESSOR KAPTEYN'S INVESTIGATIONS. 



Professor Kapteyn, Research Associate of the Observatory, has con- 

 tinued his investigations of the arrangement of the stars in space. 

 A first approximation for the solution of this problem was given in 

 Groningen Pubhcations, No. 11, and in the Proceedings of the Amster- 

 dam Academy, March 28, 1908 (p. 626). In this first attempt the 

 evident differences depending on galactic latitude were neglected and 

 only the average stellar densities at different distances from the Sun 

 were derived. 



A second though still provisional approximation has now been made 

 in collaboration with Dr. van Rhijn and published in Contribution No. 

 188. In this paper the different galactic latitudes are treated sepa- 

 rately, but the differences in galactic longitude arising from the star 

 clouds of the Milky Way are still disregarded. 



The investigation leads to : (a) A more complete knowledge of the 

 luminosity curve. This has now been determined for a range of 22 

 magnitudes, for 18 of which the accm-acy must be high. The curve 

 can be represented by an error curve with astonishing closeness. The 

 median (or average) luminosity of the stars is found to be 2.9 magni- 

 tudes fainter than that of the Sun. (6) The change of star-density 

 with distance from the Sun for different galactic latitudes. Repre- 



