MOUNT WILSON SOLAR OBSERVATORY. 221 



(6) Apparent magnitudes and spectral lines being the same, the 

 stars are redder the farther away they are. 



If these phenomena are real, as the existing evidence strongly 

 indicates, their importance can hardly be overestimated, whatever 

 be their explanation. The evidence seems to show that the effect, 

 though small for the brighter stars, becomes far greater for the very 

 faint ones. The quantitative investigation can thus be undertaken 

 to the best advantage with the most powerful instruments. It must 

 embrace not only the definitive establishment (or refutation) of the 

 observed facts (a) and (b) , but also — in case of confirmation — the deter- 

 mination of the question as to which of the possible explanations is 

 the true one. 



Professor Kapteyn and Mr. Kohlschiitter have also undertaken the 

 investigation of the luminosity curve for stars of each of the spectral 

 types F, G, K, M. For the B and A stars such a determination will 

 form part of Professor Kapteyn's study of these stars. In fact, for 

 the B 0-B 5 stars the luminosity curve will be given in the paper alluded 

 to above. For the F, G, K, and M stars, however, another method 

 must be employed. The great increase in the data for parallax, 

 together with the excellent data for proper motion at present avail- 

 able through Boss's "Preliminary Catalogue," offers good promise for 

 a successful treatment of the question. A prehminary solution for the 

 K stars has already been obtained. The work is meant as a first 

 step in the investigation of the arrangement of stars in space, for 

 each spectral class separately — the most urgent problem at the pres- 

 ent time in the study of the structure of the universe. Of course a 

 reliable solution of the whole problem will not be possible before 

 much fainter stars than are now available can be included. It is 

 hoped that this preliminary work will greatly help further study, by 

 aiding the observer in deciding as to the kind of observations and the 

 selection of objects required for a future successful treatment of the 

 problem. 



Absorption in Space. 



Mr. van Rhyn began in December a series of observations with 

 the 60-inch reflector for the purpose of determining whether the 

 fainter stars are redder than the brighter ones. Regions surround- 

 ing a bright star with known color index are photographed in the 

 principal focus on an ordinary Seed "27" plate and on a Cramer 

 isochromatic plate with a yellow filter, the latter combination giving 

 approximately a visual intensity curve. The intensity of the bright 

 star was artificially reduced about 8 magnitudes by means of a dia- 

 phragm and a rotating sector. This reduced intensity gives the zero- 

 point for both photographic and photovisual magnitudes. The scale 

 was established by taking on every plate two exposures of the same 

 duration, one with a full aperture, the other with a 32-inch diaphragm. 



