MOUNT WILSON OBSERVATORY. 215 



Measui-es of the color-indices of Herschel's nebulous stars by- 

 Messrs. Seares and Hubble show that these interesting objects are 

 much redder than ordinary stars of the same spectral type, pre- 

 sumably because of the scattering of their light by their nebulous 

 surroundings, though other explanations may be suggested (p. 

 236). Another investigation bearing on the nature of spiral neb- 

 ulae and the problem of "island universes" has also been made by 

 Mr. Seares. This consists of a determination of the surface bright- 

 ness of the galactic system as it would appear from a distant point 

 in the direction of the galactic pole. It turns out that the surface 

 brightness of all known spirals is greater than that of the Galaxy; 

 in some of the brighter ones the proportion is about 100 to 1. 

 Thus, if these objects are really systems of stars, they must differ 

 greatly from our own system in the relation of stellar density to linear 

 dimensions (p. 240). 



Some years agoProfessor Kapteyn derived a first approximation for 

 the laws of stellar luminosity and density. In the meantime, a large 

 amount of material has been accumulated and much work has been 

 done in the direction of perfecting our knowledge of these laws, which 

 are fundamental as an expression of the constitution of the galactic sys- 

 tem. Much still remains to be done before a complete solution can be 

 given ; but a second approximation has now been finished by Professor 

 Kapteyn, Research Associate of the Observatory, with the collabora- 

 tion of Dr. van Rhijn. It is still necessary to treat all spectral types 

 together and to regard the system as symmetrical about an axis, with 

 the sun at or near the center. The luminosity curve has now been car- 

 ried well past the maximum, which corresponds to an intrinsic bright- 

 ness 2.9 magnitudes fainter than that of the sun. A Gaussian error 

 curve represents the distribution of the luminosities with an extraor- 

 dinary precision over a range of more than 18 magnitudes. Professor 

 Kapteyn' s investigation has made it possible to give for the first time an 

 indication of the variation of stellar density with galactic latitude, as 

 well as with distance. The results are most reliable in the direction 

 of the poles of the Galaxy. In this direction the density falls to 0.01 

 of its value near the sun at a distance of about 1,200 parsecs, while in 

 the galactic plane this value occurs at a distance of about 9,000 parsecs. 

 The curves of equal density are much flattened and emphasize anew 

 the structural importance of the Galaxy. A full account of this in- 

 vestigation appears in Mount Wilson Contribution No. 188 (p. 254). 



Planetary nebulae, especially on account of the resemblance of their 

 spectra to those of Novae and variable stars (p. 247), are of great inter- 

 est in the study of stellar evolution. The extensive investigations of 

 these objects at the Lick Observatory render another general attack 

 unnecessary, but there are certain particulars in which we may usefully 

 supplement the work at Mount Hamilton. Mr. van Maanen's meas- 



